I only played maybe 10% of the games in my library (of around 200 games) and I think that's fairly standard for Steam players. I think you are very reasonable with your selective purchases, even if you don't play all of them.
Yeah, lots of teamkills happening in L4D matches when you play with randoms. I played it for endless hours when it released on XB360 a few years back. It's a bit expensive at 20 so yeah, sale it is. I will message you when I get it and then we can blow open some zombie heads. ^^
Been trying to play more of my Steam library, slowly getting there.