I LOVE map editors. Started with the cheezy thing in Warcraft Orcs and humans, went crazy on warcraft 2...Then starcraft ahhh that was the first truly awesome editor, warcraft 3 followed with a pretty similar editor. I love the customization! I just hope they release the editor a bit before the game and let the community get started, there's work to be done!
Another thing I think would be great is a simple way to do full on campaign mode custom maps without the usual string 1 map after another, but creating it to mirror the actual campaign, with possible deviations and different paths. I.E. you beat a map 1 way it goes to next map, but if you beat it another it brings you to a different one. Campaigns just own!