Cena's an odd case. Everything I've read about Cena the guy indicates he's one of the best people in wrestling, he does a ton of charity work and appearances that never get made public because he didn't want it publicized. What sucks is that WWE pushes him down people's throat and it turns into people hating Cena personally instead of how the company books him. He's really just getting the same push Hulk Hogan got in 1984, with the difference is that Hulk had more charisma than practically anyone else in wrestling history to make up for his five moves and Cena doesn't. Even then, what charisma Cena does have gets hamstrung by the asinine material he's given most weeks. That's part of what made this past Raw good: they let him be John Cena instead of reading a script (not in the "he's shooting" sense, but by not making him read promos verbatim).
Guys who were great promos like Flair, Savage, Piper, Austin, Foley, Rock, and HBK were never forced to read long winded scripts. If they would have been, they probably wouldn't have been the greats they became. At least 75% of why young guys in WWE suck now is because they are all given scripts written by the same people to read word for word with no room for improv or creativity. How did CM Punk break out of the pack last summer? By delivering a promo he came up with himself that was so different than Stock Wrestling Promo #70889 it made people sit back and go "holy shit, where did that come from?". Because they let him be himself, and it struck a chord with people. There will always be guys who need to be scripted, and should be allowed the chance to have someone write stuff for them, but people who can talk and come up with great promos on their own shouldn't be muzzled from doing so.
John Cena sells merchandise better than anyone else in WWE, the kids (ie: the next generation of fans) love him, all the business indicators agree with him being the franchise player and he should be pushed accordingly. However the booking does him no favors, and at times almost seems purposely done to make certain segments of the fanbase sick of him. My biggest beef with Cena has always been how he doesn't seem willing to use his veto power with Vince. I'm not saying he should be like 1990s Shawn Michaels and refuse to do anything he's asked to do, but John doesn't refuse enough. I've never seen a top guy get booked in so much shit. I don't know what I looked like in writing, but the whole mess he just did with Kane is a prime example. Both guys really tried their asses off to make it work, but the whole storyline was just ri-god damn-diculous. I don't think anyone would have begrudged Cena if he would have just taken Vince aside and said "c'mon man, I shouldn't be doing this bull shit".