Reasons to love (or at least respect) Cena in my opinion:
– He’s a good worker, not a great wrestler but certainly not incapable.
– He’s capable of being carried to great matches by really talented workers, something not everyone can do.
– He himself has carried a lot of people (most obvious example being Great Khali and also a very green Lashley) to matches better than were expected.
– He’s good at improvising when things go wrong, the best example I can think of being the TLC match against Edge where the table got knocked off the other table and, rather than just leaving it and climbing the ladder as planned or stacking them again without trying to explain why (thereby making it painfully obvious a spot was about to take place), he took the time to make it look like he was in a dilemma between putting Edge through the tables and trying to win the match. It was still clear they’d fucked up, but it was good to see someone try to work psychology into it especially given what spotfests WWE ladder matches often are.
– He puts up with a lot of shit from the audience and, rather than cry about it, uses it to his advantage. The One Night Stand match was a great example, he came into it with WWE trying to book him as a cookie-cutter babyface but he knew damn well the fans were going to boo him out of the arena, so he basically played the heel for that one night. Compare that to Batista a few weeks later on Sci-Fi who had no idea how to respond when the ECW crowd got on his back, he had no plan B and acted like a baby about it afterwards.
– When he needs to put someone over, he does so. And I don’t just mean losing to them, I mean making them look like the better man.

Reasons to hate (or dislike) Cena:
– He’s shoved down our throats way too much.
– The whole ‘underdog coming from behind’ thing doesn’t work when he keep on doing it, especially when it’s not even like he’s small compared to the rest of the title scene, he’s a big guy who you’d expect to take care of himself.
– He got popular because of his rapping anti-hero gimmick. If he’d kept that up he’d probably have stayed over with pretty much everyone, instead they just turned him into this lame superhero who rarely does any wrong. I doubt Austin would have become as big a star as he did if he was turned full-on white meat babyface as soon as he won the title. There’s just no edge to Cena but that’s a flaw in his booking rather than Cena himself.
– The STF still sucks. He’s not the only person to do a really bad version of a submission (Rock’s Sharpshooter for instance) but it’s annoying that it hasn’t got any better yet he uses it as a finisher for many matches, something Rock never really did with the Sharpshooter.
– A lot of pushes have been killed by putting Cena in there thinking it would help them, but ended up just taking all the attention away from them (see: Zach Ryder).

I think most of the issues people have with Cena are down to the way he’s been booked and written, rather than what he does himself. A lot of other criticisms, like the repetitiveness of his matches, don’t hold water when you realise how much variety there’s actually been in his PPV matches especially, and that most of the biggest name wrestlers have the five moves of doom at their disposal. In fact, Art O’Donnell did a great job of mocking the childishness of the whole smarkish Cena-bashing culture in his induction of the Cena/Laurenitis feud. He did such a good job that many actually thought he was agreeing with it, despite the obvious sarcasm in his words.