A little bit of everything. Balance is probably most important. There has be be an incentive for a player to choose which side they want, not pick the same side every game. Terraining/eye candy is important because it helps show the effort you put into the map. Skins and models are nice, but they quickly jack up the file size, which may make it non-bnet fiendly. Add extras/sidequests/duels or other things to make it so its not the same hack and slash as all the rest. The idea it to create something fresh and new that no one else has done in an AoS. Thise types of maps get recognized very quickly.
-VGsatomi