Not only that, it got me to wondering what would happen if you used a standard RDBMS like MySQL and denormalized your data? Would it scale easily? Would its performance be good?

Yes and yes. This is pretty much exactly how we built our data-warehouse at a prior gig. You can read about it here:

http://www.oreillynet.com/pub/a/databases/2007/04/12/building-a-data-warehouse-with-mysql-and-perl.html

Of course, a data-warehouse isn't really all that similar to what you're proposing, it just happens to also be less-than-normalized. Personally I think these unrelational DBs are solutions in search of a problem - RDBMs have problems, sure, but I'm not seeing how stripping away so many useful features helps me!

-sam