I have 22 games on Steam, average playtime is 40.7 hours. 37 avg on action games (e.g. HL2, Bioshock Infinite), 41 on stealth games (Deus Ex, Dishonored), 44 on RPGs, 82 on sandbox (New Vegas, Skyrim, Amalur) and 171 for strategy (EU 4, Shogun).
Yeah, I know you were dying for a statistical breakdown of my gaming habits by genre, you're welcome. Side-note, it'd be cool if there was a stats tab in Steam to compile this kind of data for you. I'm sure Allegory would agree.
I only have two Steam games I've never played - Darksiders 2, bought bundled with #1 which, it turns out, I didn't much care for; and Spiral Knights, which I'm pretty sure someone gifted me out of a multipack and which we never got around to. Normally I give a game at least a couple of hours before I decide I despise it.
Apart from bundles, which've already been mentioned, there are a bunch of f2p games, very short indie games where 2 hours is game over; hell, Portal is what, 5 hours long? Steam also counts things like Half Life: Lost Coast (a 10 minute tech demo level packaged with the orange box), beta clients and some expansion packs as separate games. It'd be interesting to know how much money most people spend on unplayed games.