How he was booked was one of the main hate factors against him at first. 2 years in the main event scene where he constantly had the title, would never look weak against someone, and wrestled with the bare minimum amount of actual wrestling moves, and ones that where full of showboating and take the minimum amount of 'skill' to perform. Yes it was very easy to hate him, myself included.
Now a days its a completely different story. I actually have an unlimited amount of respect for the guy. He is a business guy, the only reason he was on top is because he took the company and carried on his shoulders like no one else. Have you ever seen and interview with the guy, he sleeps 4 hours a night, every night, he wrestles 4/5 times a week, does as many media appearances as he can fit in, does movies on top of wrestling full time, respects the fans who helped make him, and he still loves his job! He is superman, they booked him as superman because that's exactly what he is. But even old arguments that people used to hate him for don't work any more but he still gets the hate because he is marketed towards kids, but have you seen the crowd, 2/3 of the people wearing his t shirts are 20+. He barely had the title for past 2 years, constantly putting people over, but he still gets shit for hogging the gold and refusing to put people over because he is superman.
Its bullshit, Cena as a person has never done anything wrong, he has been booked as being too strong sometimes, but that's just because no one steps up and deserves to beat him. Its changed recently. He has never beaten Sheamus one on one on television or ppv, do you know why, because Sheamus stepped up, he proved he was willing to put the company and wresling ahead of himself and he gets rewarded for it. That's how these things work.
Cena is entertaining, he is also one of the hardest working people, possibly ever in Wrestling. Watch every ppv match he has had this year. They have all been amazing, even some from late last year were amazing. His final feud with Orton, they were some of his best matches, and they weren't because he was carried by Orton. The dude can't catch a break, he is extremely dedicated, works extremely hard, and he still gets shit for it. He is the only person who after not having the title for a year, can go up against a strong opponent, and when he wins because it was the right thing to happen, all the marks go 'Cena wins, big surprise there', get your head out of your arse for you will never be half the man Cena is.