Gears of War 3
IDK. The game mechanics were a little better than the previous games, but the cheap (main) character death by an unavoidable explosion lacked the emotional impact that Dom's wife and Tai's death, respectively, did in the second game, of which I believe to be the most emotionally potent moments of the series.
The hardcore mode was incredibly easy until the final fight, and then it might as well have been on insane mode. The learning curve between hardcore and insane is stark in contrast to the setting from the previous installments.
While those issues were glaring, the scenery did make up for the lack of color in previous games. The big twist in the game revolving around Fenix's father's role in E-Day was never full explored, and more ancillary information could have made the story feel more complete.
The addition of mini-game inspired levels like the submarine chapter and flashbacks from the beginning of the game was an acceptable way of introducing new aspects of gaming to a series where straight-forward 3rd person gameplay was the crowning achievement.