Because of one mechanic or another, whether that be story, gameplay, voice acting, whichever.
I'll focus on story. There's two games that are scarred deeply because of poor story telling, and one on story and voice acting.
The first, and more popular/recent one, is Assassin's Creed. I was really excited about this game...until I found out it's dark secret: It has a dual arching story. Now, done well, that isn't a death bed, but the way they did it, I was completely taken out of the experience of being a medieval middle east assassin. It killed the game for me in a few ways, but oh well.
The next, less known about but equally as awesome game, is Darkstar One. This game's gameplay is amazing, the graphics are pretty good, and the leveling system is dead on. However, everytime this bitch who's riding with me speaks I feel like jumping through my screen and strangling her until blood is slowly carpeting the floor of my spaceship. She'll say things such as (right after you kill an enemy) "See you later alligator". I'm sorry, but what is this game rated? The same goes for the main character, who is this Australian fuck who apparently went from being an insecure teenager to a pissed off, xenophobic alien murder who destroys everything in his path, while at the same time acting like a tough guy only to be pushed into meaningless mission after mission. Last I checked, he was the one with the nukes on his ship, not the retarded scientist on the research station 15 units away from the nearest police officer.
The story is equally as horrible, and I've already touched base in a way with the research station bit. It seems like this man who was a flight trainer, all of a sudden is an interstellar bad ass who apparently can avoid detection of the police even after kidnapping a researcher and murdering the main characters father. Oh, sorry, mild spoiler there but you probably would of caught onto it within the first 10 minutes anyhow, and then when they try to make it sound like a surprise you have to ask yourself how dense the characters around you really are.
It's a good game, by the way, however bad I make it sound.
So, what otherwise great games are destroyed by one aspect or another to you?