I have a feeling my opinions going to make some people mad but here it goes...
DOTA 2 - Great game, but a LoL killer? Not even close, it's been widely accessible for a long time now, and yet LoL still remains the most played game in the world.
Diablo 3 - Not bad, but didn't even try to satisfy fans of the Diablo franchise. This game will be remembered for ages as one of the prime examples of a sequel gone wrong.
Legend of Dragoon - Hyped for many years as the slayer of the Final Fantasy franchise...yet when it released, it released to lukewarm reception and ultimately failed to distinguish itself from other RPGs.
The Elder Scrolls IV: Oblivion - Dull and uninspired, the only things it succeeded in was making Morrowind look better and paving the way to the far-superior Skyrim.
Final Fantasy XIII - Not a bad game in and of itself, but yet another example of a series that deviated too far from its roots and as a result, antagonized fans of the franchise.
Chrono Cross - It had a lot to live up to, as even today Chrono Trigger is regarded as one of the best RPGs ever made. Yet it failed to capture the same charm, and took the story in a direction that almost insults its predecessor. It's not a bad game, but it's definitely a bad sequel IMO.
God of War 3 - The beautiful exterior fails to hide its simplistic gameplay and uninspired puzzles that serve as more of an annoyance rather than a satisfying break from the action.