Well, for me, a good map is...
(bad example/good example)
#1
A map with a varied height - flat plains and big mountains, as well as something in-between.
Natural barriers and chokepoints (rivers, ravines etc.). (pinpoint/vanilla maps)
#2
Little to no buildings (unless it's urban-type map).
Multiple routes so that it isn't easy to simply camp in a certain spot. (pinpoint/anticbridge)
#3
Easy for eyes. No pink-red-yellow combinations. (hallway/bridgewars)
#4
No unnatural (scripted) restrictions (ie. hurting water - unless it's supposed to be lava ie. Firestorm). Look #2. (pinpoint)
#5
ORIGINAL. Something that hasn't been done before. (bridgewars, anticbridge/deck17?)
Something like that.