The Mass Effect series.
I originally rented #1 with no actual knowledge of what it was other than some hype and a thread on my old forum saying it was great. So I went to blockbusters and it was the only real game there, so I tried it.
Went into it thinking, this'll be awesome. An hour in, waiting for awesome, 2 hours in, still waiting, 3...4...5... I couldn't figure out where the awesome was.
Maybe it was hiding, i'm not sure. Perhaps I missed it during those times I fell asleep. I dunno. Ended up returning it and saying the disk wasn't working so I could rent another game.
I could see potential for a good game, but for some reason it just never made it to one.
I properly went through #1 and 2 when they were released on PC because I still heard great things yet remembered bad things. At least 2 has funner gameplay, but I was still bored to tears with all the 'story' bits, which is odd because I typically love story driven games...when they're good. Though #2 was a bit more gameplay focused thank god. But I still kept going, hoping it would get better and being disappointed when I hit credits and it still hadn't gotten better.
Other than that, I can't think of much I stuck with if I felt it was lacking. I have so many games I just don't have time to pray a game gets better.