I can only think of one. I've got loads of games, but I've never really bought something I then didn't like, oddly. Some have taken more adjustment than others, but I've come to like them all. Except one.
Namely Warhammer Online (grr).
Not that I was too surprised. I knew it was just another attempt at challenging World of Warcraft off the throne. But it did look interesting, and I've always rather liked Warhammer, much more than its ridiculous emo sibling 40.000, at least.
What I got was the boilerplate mumopuger experience, and you could just feel how it had been carefully boiled down and tweaked to be as WoW-y as allowable. The emphasis on Player vs. Player action was interesting, but... Yeah, everyone went Chaos, and the few who joined me and my grumpy witch hunter were whiny rushers. Everyone ran off, died, and whined about it in the chat. It was like fighting alongside a platoon of sweary chickens, in short.
I could've lived with that. The Warhammer world didn't look its best through the mumopuger filter, but it could've worked. Except for one thing. When I get my fantasy mumopuger hat on, it's for the roleplaying. What is the use of building a lovely fantasy world if you aren't meant to play pretend in it, eh?
But that hope died out rather quickly. I soon understood I was the only one interested in roleplaying on the whole server, which was billed as an RP-server. Even if I hadn't been the lone, sad deviant, there simply wasn't enough atmosphere.
Most mumopugers have an option to toggle to walk instead of run, and an option to put your weapon away. Maybe even sit down by a warm campfire. But Warhammer Online did not. You could only run and scowl, weapons in hand. Even inside a cozy, well-designed tavern, it was impossible to get into the feeling because all you could do was jog about with your sword out like an idiot.
I spent about two weeks there, doing half-hearted questing and getting ganked and owned by the Chaos players. I saw not one single roleplayer. I did try to kick some RP going, but no one seemed interested. I think I frightened quite a few players off, even.
It was a very unsatisfying experience... I still haven't found a non-WoW mumopuger with tolerable roleplaying.
I'm rather glad I'm not disappointed very often, now.
(EDIT: Emphasis on "tolerable". WoW is not very kind to standards, either.)