ratrace, wizard, carousel, chillout, sidewinder and icefields were always my favorites gameplay wise.

as far as what to look for, there are many things and you won't truly understand them unless you actually spend a lot of time playing the game. some things to keep in mind though:

-ctf maps should normally have a symmetrical design, in order to the map being one sided (damnation). Slayer maps can go either way, as long as there isnt a major camping spot. There are however some cases (ratrace) where asymetrical design made it great for slayer and ctf.

-flying vehicles (banshee) are terrible for gameplay, period.

-rocket warthogs are the best thing to happen since sliced bread

-any more than 2 teleport groups are confusing, and 2 different teles should never lead to the same spot (cliffhanger). its also best to try and design teleports to be out of reach of vehicles, personally been exploiting tele blocking for years and I hate it.

-try not to create small chokepoints in big maps like grove final and wtc. as popular as the maps were, this is what really kills those maps (in a game with good people it IS impossible to cap.)

-big confusing/complex bases are (grove, wtc >.>)

-there should be less vehicles than players in your map

-tanks are also