I can only think of two right now, and only because others mentioned them here.
Dungeon Defenders: So... boring... seriously... I can really get into some tower defense, but this game is just sooooo slow paced and dry. I was really hoping that the combat would be interesting, so that your skill with your character would actually make a difference in holding back the hordes. Nope. You're basically a mobile tower. It's all hard numbers.
Arma II for Day Z: The hype got me. I knew it had a reputation for being rough around the edges, but... my god... I couldn't figure it out. I can figure out Dwarf Fortress. I can't figure out Day Z. I don't know how anyone plays it. And the atmosphere is completely inconsistent. Everything looks comically broken during the day. My wife sat next to me and we both laughed our asses off as I ran from extremely goofy looking zombies. At night, the audio takes over and it's awesome. I spent like 15 minutes just laying down in the middle of a small town and listening to zombies shuffle by all around me, wondering when one would stumble over me. You have no idea how much I want a Day Z done right. The worst part is this is the most money I've spent on one game in the last couple years, and it was even on sale. I just don't understand how this thing reached such a godly level of hype.