Because I have a limited budget for games, I only get one every few months. That gives me plenty of time to research games that I think I'll like and it has proven me well. I dont regret Dragon Age 2, I dont regret WoW or MW2. But I do regret however..
Far Cry 2
I bought this on a whim since I liked the first one, and it seemed interesting at first. Many hours of boring missions tied together with no narrative later however and just as I thought the end was thankfully in sight, they open up an entirely different sandbox with the exact same structure and lack of plot as the first sandbox. I finished the game, but I could only play it in short bursts or else I would fall asleep. Eventually I finished it, and to this day, I have not played it since.
The honorable mention thought is when I bought my little sister a game. She already had Viva Pinata, and though Viva Pinata: Trouble in Paradise would be an ENTIRELY new game. It wasn't, and she only ever played it once. She wont even let me trade it in for another game because its hers. Yes, I did buy it for her, but does it matter? She will never ever ever use it again.