How playable is a map (and NOT, how beautiful it is made)? A good map brings players to play with or against each other, it does not confuse, does not bore, if your are into playing, and not into wondering or even being annoyed by the map, it is a good map.
I compiled a number of characteristics I think are distinctive.
positive things:
a + Can it be played well with many players (15 on each team, 30 total)?
b + Can it be played well with few players (3 on each team, 6 total)?
c + Does it have a clear layout (=you don't get lost easily after a few games)?
d + Is it not too hard to move to another base location than default?
negative things:
z - Does it encourage players to jetcamp?
y - Does it have many long corridors?
x - Does it have exactly one base location far superior to any other?
w - Does it have no really usable base locations at all?
v - Does it have dead ends only accessible to small aliens and crouching humans?
u - Does a map allow "speed build"?
These are the points out for dispute, maybe they could be weighted, feel free to comment/add, here are my thoughts:
a) Do you know the feeling to step on each other's toes constantly, even if you try to avoid it? Such maps are not good for a big number of players, and thus reduce the chances of a good game by the chance of having too many players in the map. [example for bad map: uncreation]
b) A good map is always fun. Even if there are only 3 people on each team. If a map is too big or too complex, you spend your time running around without ever confronting each other. [good maps: atcs, tremor]
c) Can you find your way, even without spectating the map for an hour? Is it intuitive to use? No? Then it is a bad map, for it makes gameplay uneasy, betraying the purpose of a map. [good maps: atcs, tremor, uncreation; bad: niveus]
d) Can you reach any other base location with ease without the help of a second builder? Don't get me wrong, a superb base location should not be too easy to reach either, a good game needs challenge. But it should also not be nearly-impossible to do. [maybe a bad map in this respect: niveus]
u) The build timer is there for a reason. If you just have to jump over an edge to suicide and happily can build again after respawn, this reason has been compromised, and the map designer did something very important wrong. [seen on: uncreation, transit]
v) The game is over, aliens have destroyed the human base. One human is hidden in a small vent, with armour and a saw. Result: It takes ages and many many dretches and basilisks to get him out, leaving the whole rest of the team in this unpleasantly boring state of watching the routine of putting the human out. [arachnid, karith]
w) The capabilites of a good builder should make a difference. If you never played a game on a certain map with a base that could at least withstand a minute even if only defended by one single somebody, the map designer failed in that respect then. [maybe alien buildings on nexus6 is an example]
x) Good base locations encourage teams to move, however, available base locations should not be TOO good. "Go to superb base location. Game over." This should not happen. Window room on Niveus or the elevator room on Karith Station are good examples, turning games on such a map into a boring routine easily. [karith, niveus]
y) Diversity is the name of the game. Long corridors are good for humans with their long-ranged weapons, and long corridors are a good thing - if there is not too many of them. Otherwise, the game may become unbalanced if even tyrants can be taken down by two stupid riflemen standing on the other side of the corridor completely locking maybe large parts of the map. [perfect example: uncreation. also a number of custom maps are plagued by this.]
z) Large rooms with high ceilings or skies? This introduces jetcamping, and making a game more a pain than a pleasure. [karith; extreme bad map in that respect: uncreation]