768x90 adsense unit just below the submission title alone would probably make reddit instantly profitable.
and that doesn't take into account the premium google adsense ads.(imagine if a random #1-#5 url title from the top was an adsense ad?)...one that had the exact font style as regular reddit ads. Those just browsing, wouldn't even notice the little gray "powered by Google" text.
then add CPM based ads...maybe something that would play on mouseover...those tend to bring in the big bucks.
or just sell advertising directly for some brand awareness campaigns. Honda, Walmart, McDonalds.
And boom...you got a profitable "community".
So there is no reason for Conde Nast to sell reddit(especially at a loss)...if they were interested in generating more revenue, they could do it at a flip of a coin.
Sure it would piss off a lot of users...but it's not like people would abandon a site, just because there are a few more ads present.