The Half-Life series, for me, at least, has been over-hyped. They're not bad games by any stretch of the imagination, mind you, but I remember that everyone was telling me at the time how awesome the game was, but after I tried it, I got the sense that "well, that was cool", but never quite anything else. There wasn't a massive "Wow!" factor that I get from some other games.
The Mass Effect series, too, as much as I'm loathe to admit it, also has been a tad underwhelming. Don't get me wrong, I love the games to death, but I have to admit that as I've played them, I've become quite familiar with nearly all of the cracks and imperfections in the fascade, so to speak. I'll say again, I love the games, but that also means that I'm aware of what they do wrong, too.
Borderlands is another game I had really high hopes for, but then was mildly disappointed. Once again, I like the game itself, it's just that there's things that I was kind of hoping would work out better, or be there at all.
I could go on and on, but I feel that I should end soon, so I'll just finish up with the latest game on the list: The Old Republic. Again, by no means a bad game, I absolutely love it to death and still play it, but it was hyped up as this massive experience that it never quite delivers on. That made me a bit sad, but not enough to ruin the game for me, thankfully.