- 2nd Edition AD&D. Played 1E for years, played 2E for years, just got fed up with the clunkiness of the system. 3E brought me back into the D&D fold, after a hiatus of probably 5 years.
- Star Fleet Battles. It wasn't the insanely complex rules that got to me, it was the twerp who taught me the game. Every time we played, he'd triumphantly pull some obscure rule (that I'd never heard of) out of his butt and use it to mop the floor with me.
- TORG. Great setting, lousy mechanics. Too often, it'd seem like combats would just drag on and on, until either (a) one side or the other got a run of great dice rolls (the mechanic seemed to be too highly dependent on spectacular results to get anything accomplished), or (b) the bad guy finally ran out of Possibilities, and could no longer cancel out our hits on him.
- Top Secret. Much the same issue as TORG. The system was an actuarial's dream. "Levelling" up a PC after an adventure was insanely complicated.
- Magic: the Gathering. I had fun playing with my friends, but wasn't interested in tuning the *&%$ out of a deck to be able to compete in tournaments. When some of my friends moved out of town, I stopped playing.
- Star Wars CCG (Decipher). My Magic-playing friends were never particularly interested, and the few times I played in more competitive settings, my "fun" decks always got spanked.
- Strat-o-Matic Baseball. I used to play this a ton. The 1994 baseball strike soured me on the sport in general, and I never went back to the game.