I'm with those who say that art is a big factor. I own a lot of non-WOTC d20 product but I might own a lot more if it wasn't for the crap that passes as cover and interior art in them. Is this shallow? Maybe, but presentation is important and its just as important for me to have visual cues for my material as it is to have text information. I'd say of the third party stuff out there, only 5% has passable art and of that, an even smaller percentage is actually good or great.