Bandwidth, servers and staff. Storage is most certainly not their problem; the English language Wikipedia is only about 5TBs of text. That's cheap.

Actually, it seems like a roughly even split between salaries+wages, internet hosting, in-kind expenses, operating, travel, and deprecation (in 2007, at least, because it was easiest to find)[1]. Wikimedia may not be a publicly traded company, but they do release their financials.

[1] http://upload.wikimedia.org/wikipedia/foundation/4/49/Wikime...