That last category is really telling. Successfully integrating your tastes into the user experience seems to elude pretty much all music players that make a go at it (excluding Pandora, which does exclusively that.) I think this is largely because, not only do your tastes not necessarily match those of your friends, but they frequently don't match your own tastes from last week.

So Rdio shows a page of what's in heavy rotation amongst my friends - but it's not accompanied by any sort of indication that I'm likely to enjoy any of these. My suggestion would be to use some of that $17M to buy up like.fm and push hard on recommendations.