Obviously, the map has to play well.

But what makes good gameplay?

Several things can help. Obviously, layout is key.
If there are two rooms with two corridors to it the gameplay will get stale very fast. Players will exhaust just about every interesting option quickly, which makes playing classes like sniper or spy rather hard if they know where your coming from.

Maps that are mirrored [I.E. Blue's layout is the same as Reds] generally ensures balanced gameplay simply because each has what the other does. Maps that aren't direct reflections of the other side are generally harder to pull off, but if they manage it they can be better than the mirrored layouts in gameplay.

Maps that do something interesting are also fun. As long as each team remains equal, a map that takes place say, on the side of a cliff, with primarily vertical fights could be very fun. Or in the case of say, Convoy, which took place on two trains, was also pretty cool and did something new. [For TF2 anyways, its apparently also a remake map, but it was fairly well pulled off]

And aside from this, if the map just flat out looks nice, that can win over many players. Not everyone wants to play on a blank orange or grey map. Alot of folks enjoy TF2 for the frankly awesome looking characters and backgrounds, its like, up there on the list of top 5 reasons they might be playing.

So look for some of these things when your checkin out maps

*S*