Sanderson has proven himself more than, say Abercrombie. The latter only just came out with his fifth book. Sanderson, meanwhile, has written 8 full-length novels (including the last two WoT books) as well as having written a few young adult books (his Alcatraz series). So I don't think that's the issue.
I think the criteria is more to do with how many active threads related to a particular author exist at a given time. It's sort of a way to gauge the popularity of said author on the Chrons, before deciding whether it's worth giving him his own sub-forum.
Yes, when I saw this thread I thought of Ambercrombie as well. Thus far I've enjoyed Ambercrombie more (the oddities and mannerisms of his characters), but that's just personal preference.
I'm not sure how exact or scientific the criteria is for granting authors their own forum here. But it certainly brings many inconsistencies.
I need to try Sanderson again. I found the first Mistborn a little over-long and unfocussed at times -- as an author myself, I'd say it could've been shorter and more tightly plotted. But I enjoyed it very much. An undemanding, fun read. Unfortunately I kept hearing people say that parts 2 and 4 and disappointed, which put me off.
I really shouldn't pay so much attention to what others are saying!
Beyond that, Sanderson seems quite internet savvy. I've seen many youtube interviews, and it's good to see that he's a really nice-seeming fellow.
Coragem.