For me out of about 170 images
Bigstock Photo 90% approved
Istock Photo 60% approved
Dreamstime 72% approved
Featurepics >95% approved
Stockphotomedia >95% aprroved
123RF 75% approved
Fotolia 75% approved
Personally I think iStockPhoto review is unrealistic. I don't mind it if they reject my images because they think they are horrible or what ever else. But often they reject them for noise issues or focusing issues, and they always noise free and have a dead on focus. But I've already ranted about that so I won't start here again.
Mark