I have a hard time believing someone can understand how to play RE6 and -not- have a blast with it. The game is
amazing once you understand the core combat and mobility mechanics, and develop some mastery of them. And the level design is fine, and quite varied. Not sure I see the complaints there.
Among people who actually know how to play the game, I think RE6 is polarizing with its overambitious variety. The game throws so much at you, and so often, that it's inevitable not every person will like every set-piece.
Dusk Golem did an excellent job describing the variety. I'll paraphrase some of his examples here:
One exhibit is Jake's Ch. 3, which starts off with a stealth sequence where you're unarmed in a medical facility. The place is sterile and bright white, and feels unlike any area in the series in terms of atmosphere. Then you enter an oriental mansion that you can explore freely, searching a variety of themed rooms for keys needed to move forward (including optional ones for a secret weapon), engaging in light skirmishes along the way. Then you're running and hiding from a tank that bursts through the wall and stalks you through the courtyards. Then you're escaping the mansion on a motorcycle with enemies in hot pursuit.
Another exhibit is Chris' chapter on the aircraft carrier. Above deck, it's reminiscent of the factory/ship levels in RE5. Below deck, it's reminiscent of Revelations 1, with a pretzel-like network of dark hallways and rooms, and a regenerating enemy that can kill you in one hit. The enemy stalks you as you search for the way forward. And then after that you're flying a jet while Piers runs around the ship avoiding a rampaging giant.
The game is constantly switching gears, and someone may not like all of those shifts. But I commend the game for its variety. It's certainly never boring.