Halo 2 - Its multiplayer portion was revolutionary, but the campaign butchered everything that was great about the original: No sense of isolation, little to no room for exploration, no immersion as the covenant absurdly started speaking English, the story was tailored to fans of generic sci-fi action movies, the violin was replaced by an electric guitar, vehicles were seemingly made of plastic, and the list goes on.
Dead Space 3 - The transition from horror to action had already started in the second game, but Dead Space 3 got rid of the final elements that made the original so great.
Fire Emblem: Awakening - "Pair up" ruining the gameplay, horrible difficulty balance, pre-pubescent characters wearing close to nothing, atrocious character design in general. This game supposedly saved the Fire Emblem series from extinction, but as far as I'm concerned an honorable death would have been better.
Star Wars: Battlefront (2015) - Does not even remotely compare to its prequel. Saying that the game lacks content would be extremely generous.
Super Smash Bros. Brawl - The presence of hitstun-cancel, hitstun being ridiculously low to begin with, by far the worst balance present in any smash game, tripping, infinite air-dodges. Smash 4 took some steps in the right direction trying to rectify some of these issues, but as of right now the series beyond the first two entries is unplayable with items off due to Brawl's legacy. Creative combos and general exciting gameplay is reserved for Smash 64, Melee and the Brawl-mod Project M.