You offered the idea that perhaps people don't like him because we can't relate to him. While this is a fine idea, can we relate to any of them? They're bigger and tougher than we'll ever be, all have more money than us in their banks, they appear on television half naked every week and... well... they're wrestlers. We have nothing in common with them.
I do not like John Cena. I don't hate him, I don't wish physical harm on him, I wouldn't threaten to riot if he wins a match, I don't think the WWE would be better off without him and he's the worst thing to happen to wrestling in the history of everything. I just don't like him.
Professionally I think he might be the best guy in the industry, with his work ethic, passion, commitment yadda yadda yadda. He doesn't choose his storylines, he doesn't pick his match endings, he just turns up, does as he's told, and runs with it. I respect the hell out of him for doing all of these things.
But I do not care for him. I don't like that we know he's going to win every single time when it matters. He'll lose every so often, but he's going to overcome the odds, hit the attitude adjustment on someone you wouldn't think he could lift, retain or recapture his title, and then cut a promo about his morals. I can't fault said morals, in the same way people can't fault CM Punk's, but it just doesn't do it for me. I'm a grown up, I know that fighting fair, having courage, never backing down, and being loyal are good things. I don't need him to tell me that. I don't watch wrestling for life lessons. I watch it to see large men beating each other up.
Now, yes, Cena does beat people up, by the truck-load. He's probably pinned or tapped out more people since he broke in than anyone else in that same time period. He kicks ass and takes names. But he does it in the same way every damn time. He wrestles three kinds of match. One against giants, one against tag teams, and one against regular guys. They vary very little. I just feel like I've seen everything he has to offer. Yeah, he's strong, it's impressive when he hits the Attitude Adjustment on 400+lbers, and he has charisma and passion that drips off him inside the ring, but he's the definition of a straight-forward wrestler. When you've seen Kurt Angle put on a clinic against Chris Benoit, or Undertaker vs Shawn Michaels at back to back Manias, or the countless other main event names who are better at wrestling than him, you just don't really have a desire to watch him.
Kids don't care about matches. They want to see an entrance, a finishing move, and a celebration. They want their hero to win and they don't care how. To them it doesn't matter how limited Cena is, because he's Cena and he's their hero. He isn't my hero, he's just a wrestler, and he isn't a great one, which is unfortunate, because if every guy was as passionate and committed as him wrestling would be epic.
So yeah, to sum up, I'm not interested in his moral righteousness (I personally gravitate more towards anti-heroes and darker characters anyway), I just want to see him wrestle, and his ability is far too limited for my liking. I want to see variety in both storylines and wrestling ability. All he does is feud over the title because it's a representation of being the best and that's his thing. His feud with Batista was good because it was one of the more personal ones he's had, it would have worked without the belt, because it was about Batista's dislike for him as a person and his jealousy. I'd like to see him enter into more varied feuds and I'd like to see him wrestle a different kind of match, pull out some tricks we don't usually see from him, change up his game plan, just SOMETHING DIFFERENT!
I dislike his monotony, that would have been a quicker way of saying it.