Deus Ex: Human Revolution for me too. Seems to be quite common for people to start it and get sidetracked. It has perfect breaks in the story that cause that I think for me it was after finishing the first Detroit sections and heading off to China. Ended up stopping to play something else at that point and never went back. Really need to get back into it soon.
Another one was Half Life 2. Got through Ravenholm but couldn't force myself to go any further. Despite all the acclaim I found it one of the most sinfully boring games I had ever played. Perhaps I just got to it too late I dunno. I'm sure all the physics were awesome when it first came out but when I played all I kept thinking was "Yes I get it Valve, you made a physics engine well done now give me some actual game please." I have been told that the game picks up later on but honestly if I'm still bored a few hours in then clearly something is wrong.
PSN ID: todd360 Currently Playing: Okami HD, Uncharted: Golden Abyss, Uncharted: Fight For Fortune
100% completion wise?: Some games here and there. Don't feel like checking and listing them all
Story-wise?: Only Ni No Kuni off the top of my head. I got about 75% of the way through and stopped to play another game really quick, which lead to another game, which lead to another game, and so on and so on.
I do want to head back and finish NNK though. It's a really good game.