Umm a bit of a different story but the other day i reinstalled CoH after having not played it for a couple of months (Opposing Fronts should also go on the list but that has improved with patches) anyway the patch system completely died on me and i couldn't patch the game to be able to play online after uninstalling, reinstalling, redownloading multiple times it totally put me off playing the game
Other than that most sequels that dont come close to living up to the originals, deus-ex being the main contender for this award