[Travis]

Interesting point. But those "features" existed (albeit in a mediocre form). So maybe "implementation of features" or "quality of features" sells a product...?

Speaking just for myself: I don't use SO (or reddit) because of the features they offer. I use them because of the communities involved. I think that's the point here -- there are loads of sites that can compete on features (reddit, meet digg...) but lack something else. It's that "something else" that drives success (partially; I'm not suggesting it's entirely based on some esoteric quality, but that the specific quality can be different for different applications).