The later versions have stuff like halftones and stuff. I mean, I guess you could do that with Photoshop too, but usually you have to create your own tones from scratch. OC has some default stuff that's easy to play with.
Watercolor brush more feasibly emulates real watercolors. I dunno, something about it blends colors and stuff together more loosely/easily. It's more fun to play around with. OC also has an option where you can connect workspaces with someone else and work on the same picture together, etc.
It's more the overall feel of using OC that's different/more preferable to Photoshop. I don't think there's much you can do in OC that you can't do in PS or GIMP, but I think it's just more fun to play around with overall.