I think Albannach has a great list here. I would simply add:

Flow - how easy it is to navigate, get to the next logical place.
Moderation - does the current system work well enough to keep this place cool
Customizations - does the current level suit everyone?

"sometimes when you make a request for the head you don't
want the big, fat body...don't you go snickering."
                                         -- Nathan Torkington UoP2K a.k.a gnat