Is it really okay to fault the game because your partner isn't too sharp?I can completely see why. I didn't even finish the coop in Portal 2. For me it's mostly just that I didn't have fun playing it with whoever I was playing with. I am pretty good at spacial puzzles and my experience has been that I had to tell my partner what to do so he can keep up. Basically I had to play two parts at the same time. That is really not what I am looking for in a coop experience.
Gears is a great choice with the campaign, Horde and Beast mode.