I think it would be good if you added support for (or just switched to :) ) starting with a description rather than starting with a drawing. I think that tends to be funnier. Especially here -- the drawing interface is annoying, so people will start with something that's easy to draw rather than something more interesting. (Whereas I think people will be more willing to draw something complicated if they already have a description. I certainly am. :) )

Edit: Might I also add, that starting with simpler things is implicitly encouraged also means you get long chains of things like "balloon... balloon... balloon...".