For example, I've had Deus Ex HR & Batman Arkham City since before Xmas and have yet to complete either of them.
You just had to mention those two, didn't you? I also haven't finished either of them yet...
Or The Witcher 2
, or The Amazing Spider-Man
, or Max Payne 3
, or Spec Ops: The Line
, or Spider-Man: Shattered Dimensions
Hey I haven't even *started* Witcher 1 yet...or Bulletstorm, Trine 2, Morrowind, STALKER SoC. Indeed, If I look down my Steam library I can easily name 12 or more games that I never finished (more if you count the Dawn of War Expansion packs!) Which is a bit weird as I like to think I usually complete the games I like, but the reality is, some just fall by the wayside.