-zf_4way_v4 is ok. It's nigh impossible for the survivors, though. Usually they get separated up too much, and overwhelmed with ease, especially when it gets to the second tier of capture points, where there's usually only three survivors left at most, and all the zombies can easily rush. It's a very interesting concept, and one that could work well with some tweaks.
-zf_alpine_b4_2_fix is irrevocably unfixable without a massive overhaul. Most rounds don't see the very first point capture due to the ease with which zombies zerg the cap. Combustible is a big one here. Survivors often get slaughtered coming right out of the gate - not that that's a terrible thing, but this map is ridiculously long, and making it so easy for survivors to die right at the beginning is bad. The first point would need to be heavily reworked, maybe put the zombies into slumbering mode or whatever the slow respawn is called, set the spawns farther back, I don't know. The second cap, that whole building, is fairly interesting, but the cap is in a bad spot; put it in the middle of that area, and it would be a lot more interesting. Third cap is good, though it's an awful long run from the second with little in the way of health packs. I'll say I've rarely seen this map won by survivors, it almost always ends on the first or second cap.
-zf_asylum_b2 is excellent. I don't think it needs any retouching. Maybe the best map; its use of lots of areas to roam, tons of entrances, and plenty of areas to set up as survivor is fantastic. Sticks to the zombie theme very well.
-zf_badwater_gov isn't terrible, but it's fairly long. Not one of my favorite maps, I'll profess.
-zf_christmas_panic_v1 this and regular panic are kind of sacred, being perhaps the most iconic zf maps. I wouldn't change much, if anything; I think it'd be interesting if the glass roof of the outside building could be destroyed, though I'm not sure how well that would work out. Might make it too easy for zombies.
-zf_cinema_a2 is new, I've only played it a handful of times, so I have no ideas.
-zf_cliff_v2_6 needs an additional spawn more than anything else; it's not fun getting so easily spawn camped as zombies. Changing it to night-time and making it all spooky would be more appropriate as well.
-zf_compound_b5 is fairly good, though it suffers from this thing where zombies can immediately get out of the spawn as soon as the round starts, and slaughter the unsuspecting survivors who spawn only moments away. The building near the zombie spawn is somewhat easily camped by survivors if they all sit up in there, especially if they have heals.
-zf_cp_gravelpit is just bad. The original Gravel Pit was bad. This is no better.
-zf_cp_volcano_b1_c is...meh. Nothing really wrong with it, but it's kind of boring, which is a shame considering how interesting it looks. The lava is a very interesting element, but I feel like it's not used to its full potential. The second cap is boring. There's also that big extra room with unbreakable glass that overlooks the outside and...serves no purpose.
-zf_cube_v7 is PHENOMENAL, easily one of my favorites. A very intense concept, as the survivors have to desperately fend zombies off until the doors open up and they can join up together; meanwhile, the zombies either drop in from above, or similarly wait for doors to open up so they can chase down trapped loners. I feel the doors closing on and killing players is working as intended, and works perfectly; having to pay attention to where you're standing, and making sure you;re not going to get clobbered by them is great. They can even come in handy if you're skillful enough as a survivor, and kite zombies onto there at the right time. If anything, I would say perhaps experiment with putting structures or hazards into the different rooms?
-zf_degrootkeep_v3w is fundamentally unplayable. I will readily admit I am quite butthurt when it comes to this map; like I said in the other map thread, it was designed for melee by both teams. Allowing for unhindered sticky and rocket spam when all the zombies can do is stand there on the cap and die is absolute garbage and completely unfun. I say take down the capture points, the main gate, and the invisible wall keeping zombies from leaping in, and keep it as a basic survival map.
-zf_desertyard_v2 is somehow fun despite all its flaws. The buildings serve no purpose besides perhaps cover for zombies avoiding sniper fire (this map is such a dreamland for snipers). The spawns are set WAYYY too far away, so that if you're not using Hunter or Leap or something, you not only have to wait for the respawn time, but then also run your happy zombie ass all the way back up to the rock. The rock is the only place the survivors every camp, anyway. The map is much bigger than it needs to be, all that room gets no use. Survivors generally just sit happily up on the rock; there's no fear, no intensity.
-zf_dustbowl_v4 is too easy for zombies, especially at the first cap. Take out the spawn on the roof, that might fix it a bit. Too often, survivors never progress past the first cap. This is not balanced.
-zf_expedition_c3 is mind-numbingly difficult for the survivors. Thinking about it, I'm not sure I've ever seen them win this. I don't even know where you'd begin to fix this one. I like the whole theme and concept, but zombies just have it way too easy on here.
-zf_farm_field_v2_a1 is one of the better maps, great use of atmosphere and fog with the size and layout of the map. Certainly more could be done with the idea, but not without taking some fundamental pieces into consideration. Fits the whole zombie feel very well, which I think is important.
-zf_fortress_v4 is very good, places with the idea of height as dominance in an interesting way. The whole time, zombies are battling to climb up to the top; if you're a survivor, falling down even to the nearest level can often mean death. It sure could use a lot of work, but it's solid. Nothing I would change immediately, though the one room is always camped, which makes it a bit boring at times.
-zf_fort_b2 is boring. Yeah, 2fort version, woohoo. The map maker did pretty much nothing interesting, just run in, grab the intel, and run out. I'd just throw this one in the trash.
-zf_halls_of_the_damned_v1 is bad, bad bad. Listen, I played Doom and Doom 2 on my old man's PC using a freaking joystick. Trick or Treat was my all time favorite level, and the sound of the Hell Baron's scream still makes my asshole clench. I love Doom. I do not love this map. It is uninspired and utterly crap; the retro facade doesn't help hide the very plain and simple and boring room layouts. I do like the use of the teleporters, and the acid is neat, and could have an interesting application. The elevator in the Tech room, and the doors themselves could be used to greater and more interesting effect. But I wouldn't even try to salvage this map; toss it in the trash, and please for the love of god take it out of rotation.
-zf_industry_v2_1b4 is a fairly fun map, though it's not designed very well. Could use a lot of tweaks, even adding and removing entire set pieces, though nothing needs immediate fixing. The "secret room" can be annoying as a zombie, especially if an engie plants a dispenser on the spawn point.
-zf_labs_ext_5b is a classic, no doubt. I don't think anything on here really needs fixing.
-zf_labs_remake_b4 is definitely good looking, and solid to play; except for the parts that were changed! The fourth point is too defensible in my opinion, with only one place for zombies to come from. The fifth point is just absolutely awful; mostly because if survivors make it there, they've won, because the zombies spawn so far away. The point itself is very boring, as well, just a small, open area. Fix the spawn for the fifth point, and I think it would be a lot better.
-zf_lake_v4 is somewhat a classic for me, though it certainly has an abundance of problems. The glaring skybox has always bother the hell out of me; does anyone like playing with that glare? There's health and ammo on every rooftop; Black Box soldiers dominate with ease because of this, just rocket jumping back and forth. Medic ninjas can be irritating too, and though that's usual on most maps, it's especially true here, for the same reason soldiers easily stay out of harm's way. The buildings don't get much use in my experience.
-zf_lumberyard_v17 is unabashed garbage. Toss it. There's nothing to fix, it's just a piece of poo. No, removing the sentry won't magically make it fun; it's still a crappy, tiny little map, with very little room to play with.
-zf_maze_v4_5 is crap. Everyone hides in the one little structure, or Black Box jumps around. Boring boring boring. I'm not sure what the map's author was going for, but there's very little of interest on here; all there is to do is for survivors to camp up, and zombies to zerg. No strategy, no fun.
-zf_mountainhome_b2 is great! It's like panic_apartment! Only not nearly as good! Too small, really. The big, flat roof makes it kind of boring; I'd add another floor. There's nothing very distinct about this map, either; definitely needs something to spruce it up. Maybe fill it with fog, like farm_field?
-zf_office_v1_3 is pretty bad. Survivors usually camp in the bathroom, or occasionally the board room. They have little reason to do anything else, which makes this map really boring. Badly designed from top to bottom.
-zf_outsetisland_v1_fix isn't bad, though it doesn't have much going for it. That well has to go, there's too often medics and pyro or something shoved in there with no way to get them out. The map is also too bright and sunny; make it night and spooky, and it might help. The map is kind of directionless, there's not much to it...I feel like a lot could be done to make it more fun.
-zf_panic_apartment_b3 another iconic map, it's absolutely fantastic. Plenty of places for survivors to hole up and run around, lots of strategies to try. Too often, though, everyone just camps out on the outside area, which is very annoying as zombies, and boring for survivors. The third floor is also kind of a mess and gets no love, while survivors often sit up on the roof, where the wide open space allows them to deal with zombies before they can become a threat. My favorite thing about this map is the numerous entrances, and how there's always a turn here or there you can take and get away.
-zf_panic_mansion is garbage. Zombies can cap the second point before the first and win; therefore the only thing for survivors to do is sit in that shitty little room while zombies pour in. Map should be renamed zf_shitty_room.
-zf_police_station_ii is pretty crappy. Survivors either hole up in the church, which is nigh impenetrable, or in the station itself, which is also nigh impenetrable. Or that shitty little hut, sitting up on the top behind the fencing...which is nigh impenetrable. All with absolutely shit-awful spawns. This map is not good at all, and I don't think anything would save it.
-zf_rapture_a2 is actually fairly solid. The multiple optional caps to improve defenses is a fantastic idea to play with, and I think it works well most of the time. I'm not even sure I'd change anything, though the zombie spawn is irritatingly far away.
-zf_ravine_b1 is simply awful. A single capture point in a tiny, tiny map, with one room for the survivors to camp. Nothing to fix, just kick this out of the rotation.
-zf_stinks - lol.
-zf_topsecret_v2_1 just isn't very good. Survivors almost always - if they're smart - camp above the breakable stairs and from that point on are usually untouchable. Besides that, the map is just too small, there's too few ways to get around, and the zombie spawn is pretty bad. Oh, there's also the fact that the door that keeps zombies in their spawn can be shot through, meaning survivors and zombies can kill each other as soon as the round starts. I don't know what I would do here that wouldn't require some fundamental changes.
-zf_tunnel_base_v2 is laughably bad. Zombies usually can't even get the first cap. Even if a spy sneaks to the last point, it only takes one or two survivors to kill him and get rid of him. Map is too small, poorly designed, zombies are made to be weak and ineffectual...it's not good.
-zf_underbase is too big, period. Zombies either spend all their time running around, trying to find their way around, or trying to get at the survivors in the one huge room with the shipping containers.
zf_volcanoevac_v7 is pretty good, if a bit hard on survivors. Nothing I would change.
-zf_yard_v4 has one big glaring problem - the big freaking ramp that everyone and their mother camps on, besides the couple of dolts that actually stick to one of the secret rooms. Make that ramp less reliable for the survivors, and it would definitely improve the map.
If I were to recommend maps to take cues from, I'd say asylum, panic, panic_apartment, and labs.