What we've found with doing anthologies ourselves (we literally just announced one today), is that you can see a lot more of what artists have to offer, and give them a bigger chance of being published.
As we've tried to find anthologies to enter ourselves, there's a ton of ways you can go at it: from a single writer thinking up a world setting and letting other artists and writers fill it up for themselves, to just having a simple theme for artists to follow. (Like we are too)
Last way I saw someone do it, was with a writer's short stories being drawn by several different artists into one big book.
All of them are very valid and damn good IMO
Honestly putting together a webcomic anthology series would be AWESOME in my eyes, and give a lot of new viewership to webcomic creators. If you're thinking of starting on this, please do! You'll make a lot of people happy I think. 