Having worked on the early production stage of Blizzard's next MMO, concept art definitely seems like the most fun part of the design process, for both the artists and those who commission them. With concept art work, you can find mind-bogglingly talented people who, when you give them ideas and let them roam free, will do these face-meltingly awesome concepts that inspire you so much you just want to make huge posters of them and jump inside.
People just get so excited hiring amazing artists and seeing what they can do with few limits in place. That's what EveryArt will really have going for it.
BTW, is there's anyone who has taken on commission markets for musical groups? Been wondering for a while: http://www.quora.com/Is-there-a-service-that-allows-people-a...