I think retention could be improved with a little work. I'm a 400 hour pub player and I don't really know where to go next. I'm always one of the better players in pubs but how I take it to the next level is unclear, be it comp or whatever.
My point is not really about me as such, it's that I don't think the game guides you in this. New players just see quickplay or whatever and are not really informed about what else they can do to really dive into the game. The result is that I imagine people get bored of casual pub games and as their interest wanes, they stop playing the game. Essentially Valve should improve the visibility of comp or other stuff. Imo anyway, maybe Im wrong.
As has been said, Valve needs to do it. The community is pretty good usually.