I feel like with every map, there's something about it I don't like, but there are some that stick out more noticeably. (That, and without variety, what would MM8BDM be?)
I'll try and draw some attention away from MM7 maps as well as maps that have been repeated over and over.
MM1ICE - I feel like most of the fighting goes on in the upper part of the level, which leaves bottom part is underutilized. A possible solution is to level out the ground a bit underwater and remove the pillar in the center of the bottom level.
MM1TIM - Aside from the conveyer belts, I think one issue MM1TIM has, is the low amount of escape routes if you get stuck in the lower part. It shares a little bit of the crashman's problem, and I think it could be fixed by putting some more ladders in. Two here, and another on the other side of the conveyer belt for the other room. These ladders would be going towards time slow and water wave.
MM2BUB - I honestly like this map for the labyrinth fight, but the fear of hitting a bomb is present. Perhaps instead of at the ends of corridors, put them on the ceiling? If so, the ceiling height for each hallway would have to be increased slightly, and right next to the intersections should be some breathing room with no mines. It would also be canon with MM2.
MM3SPA - In most cases, this map feels too small and clustered. I like the tower in the middle with the 2 levels, but maybe expand the map out? (I'm thinking of a design similar to the upside-down wily level in one of the map packs)
MM5WAV - Once again, a map I like the gimmick, but I feel like the map is about 50-50. (half land, half water) What makes this bad though, is that the land is concentrated in 3 spots where the water is concentrated in 1 spot. There's not much variety going on here.
MM5GYR - I don't like how big this map is given the fact that it doesn't use the full space. A solution might be to make Gyro 4 sections that are connected with his fans, but these sections are fairly large in size. It would cut down the map size while maintaining this map's "gimmick"
MM5GRA - Another map that the gimmick messes with the weapons. One problem I have, is the corridor with blizzard attack. If gravity is down, you're only option is to go in the teleporter, or run towards your attacker. Going along with this, there is only 2 (3 if you count gravity) ways to reach the area with dust crusher, and that's with stairs and teleporter. Perhaps a staircase that you can use regardless of gravity is added, it would be beneficial to add it to where the large energy and the hallway with blizzard attack are. (Combine the two areas with this staircase)
MM5DAR - I know it was brought up in a different topic, but there is a set of stairs that you can bonk your head on if you
strafe run. If you run straight ahead, you won't hit your head on the ceiling. It's on the side with star crash and napalm bomb.
MM6YAM - One thing that bugs me, is the platform with the w-tank on it. Because of the height difference, a normal player would not know about it. Going along with this, when you go for the w-tank, you can't see your enemies due to the cliff. Perhaps extending the ledge outwards so you can see the w-tank from above would fix this.
MM8SWO - One particular spot that annoys me, is the area with tango. Even though there is a tornado hold, there's not many other ways to escape. What I think could fix this, is having a thunder peg on the ledge above. Not only does this add another escape route, but it could be used to thunder claw up to the window, opening another escape route. (It might look silly however)
Honestly, aside from Bubble and Dive, there is no "unplayable" map. A map might be annoying, but without variety and gimmicks, every map would be the exact same.