People when talking about Cena usually miss the point. It isn't the fact that he wins all the time, because that's just what happens with your top star, it's how he wins. Typical Cena match is he gets 10% of offense in, gets the crap kicked out of him for 85% of the match, the remaining 5% he hits his five moves of doom and wins. While that would be okay some of the time it occurs far too frequently.
Sometimes it gets downright ridiculous. Most recent one I can remember was against Dolph Ziggler in early January. Ziggler hit a legit 4 finishers I think it was and Cena kicked out over and over. One Attitude Adjustment, which while not nearly the worst finisher of all time is pretty horrible considering it has the same impact power of a back body drop, and Ziggler is down for 3. It's laughable.
Then there's the character that incredibly hasn't changed in a very long time. You know, that would be fine if there was a wealth of promo material he could get out of himself but when almost every single promo goes "this is the most important moment of my career and I know I'm a longshot to win but I just have to do it" or "I may have lost but I'm going to come back and work harder than ever to win next time" it gets old fast.
What's funny is because he's the face of WWE I would love to like him. While he's not perfect and can get divaish like any other top draw has and probably has less to complain about than most of them, he is a dedicated company man who does all the great stuff for Make a Wish, he can wrestle, he can talk, and he's in great shape. I'm not committed to disliking Cena if they do something interesting with him and that does NOT mean turning him heel. There are plenty of different ways to be a face that work but the way he does it drives me crazy.