Title says it all. Say why you got it and why you never finished it or got around to it.
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I'm not one of those guys who likes to... what is the meme? "Get their wallet raped during the Steam Sale and buy a ton of games." I rarely buy games, in fact I can actually see the list of all my steam games without having to scroll. Still, there's some games I haven't actually beaten or played.
Borderlands 2: I actually played this game - for 16 hours according to Steam - but ran into a difficultly spike after The city flew away. I tried to catch up to it but the monsters started getting higher level than I would progress naturally. Wasn't able to repair the transponder/beacon (because the robots kept wiping me) and I absolutely didn't want to go back and grind levels like it was the 90's or something, so I uninstalled.
Kingdoms of Amalur: I've got several dozen hours invested in this game but have never actually finished it. Actually, I've never even made it across the ocean to the second continent. The writing is excellent and the gameplay was very sit back and smash buttons relaxingly. I don't think it was the game that was the problem, but rather there were games that better caught my attention at the time.
Half-Life 2: Yes, I'm a heretic: I call myself a member of the PC master race but have never finished this game, for the same reasons as KoA.
Transistor: Picked this up during the Christmas steam sale due to PCgamer's recommendation along with Banner Saga and Dragonfall. I finished Banner Saga and Dragonfall but completely forgot about Transistor. In fact, I didn't even know I had it until I booted up steam today. However, thanks to WoW, HotS, and Pokemon, I won't have time to play it.