Diablo 3 PC. I wish I had waited for the Console versions that include the expansion, controller support, and is cheaper than the PC version+expansion (and much more likely to go on sale than Blizzard's digital PC store). And the loot was just so mediocre for the whole time I played it unless I used the auction house which took away what little feeling of accomplishment that game gives me. It's a fun game, and the expansion/patches seemed to fix a lot of it, but I'd much prefer to play with a controller.
I don't have either an XB1 or a PS4, but Diablo 3 is hilariously one of the few games I'd be excited to pick up once I do buy one of them.
Red Faction Armageddon. After falling in love with RF: Guerrilla, I was incredibly excited for this follow-up. Turns out they reduced the destruction to a much less important part of the game (actually making it a hindrance most of the time) and making the game a boring corridor shooter with annoying jumping alien enemies. Such a massive disappointment.
Dark Souls 2. The first game took me about 5 hours to finally click with me, but once I did I loved it pretty much the whole way through (excluding the Capra Demon), and I finished it just a few months before DS2 was set to come out so I was excited to be able to play a sequel so quickly. But once I started it, it just felt like a downgrade from DS1 in every way (every way I cared about anyways) except for graphics.
The weapons wore down way too fast, I couldn't level up at any bonfire anymore and had to return to the main town anytime I wanted to, and the constant lame bosses got on my nerves very quickly. I gave it about 7 hours and haven't been back since.
Thankfully I rarely ever buy games day-one or preorder anymore (and I'm much better at guessing if I'll enjoy a game or not ahead of time) so these examples are few and far between nowadays.