I have many unplayed Steam games, largely due to buying packs of games where I am interested in a few of the ten. Then there is the Steam sale effect where you see a game you are kind of interested in playing at 75% off, so you pick it up now for potential play later.
I am thinking that Borderlands 2 has quite a bit of the latter. Looking at Steam achievements for Borderlands 2, 27% of people have not gotten as far as picking up the first gun. More than a quarter of people who own this game have not played it. 27% have completed the game’s storyline, which makes for nice symmetry. And 2.7% have been to all the named locations on the map.
I have no great insight here, just an observation. More than a quarter of sales of a AAA game did not lead to playing.