His promos are childish, plain and simple.
Surely not childish in a PG era?

I wouldn't mind the jokes so much if they were good but unless you are such or love loads of gay jokes, they aren't.
For those who think it is his rap schtick, its not since thats virtually vanished, that was actually pretty good back in the day.
I'd say the he is better than average on the mic. I just think its too much. I remember the same with HHH when he was the champ and leading the company. He used to come out at the start of RAW or SD for 20 minutes and after about 2 months of this, i got so bored of him it was untrue. In fact he bored me so much it took him a few years for me to even be interested in his work again.
When Cena writes his own stuff i think he is good, but i get the impression that the content isn't always up to him.
Now blame the dirtsheets, blame Cena, I don't know who is completely to fault, but his comebacks after beatings are often to quick.
Same could be said for lots of guys. Wrestling was created on face's taking a beating and overcoming. Cena is abit of a throwback. Very few these days are designed to 100% face as this just isn't considered cool anymore.
Bret was probably the last true 100% face to win multiple world titles. before the HBK and then Austin era took wrestling down a different road.
Its not the same now, but a few years ago he never seemd to lose, it was an on going forum joke for any feud Cena was involved in SPOILER: CENA WINS!
That was a good thing though i think. coz as bored as i got with him winning, i was continually surprised that he kept winning. I remember when Umaga first came in and quickly got a title match, i was convinced he was gonna lose, and then facing HHH at mania saw no way he was gonna win and certainly not make HHH tap. It was a real jaw on floor moment for me and for my friends. Then when he did it to HBK as well, i was shocked again.
I remember at this time WWE tended to have longer title reigns anyway (Lesnar, Cena & JBL all had lengthy reigns)
Back to his character, with the "Rainbow Coloured" gear and his promos if you over 18, (Barring the odd exception - Gnrfan
) he just doesn't have any real appeal.
One of his positives is his passion, although that can be a negative, its great he loves the company and the fans and the business and will never leave, but he seems to feel the need to tell us again and again and....
The best thing about him is he is over. Cheer. Boo. He gets a reaction, he never suffers from pin drop syndrome like supposedly over guys like Orton can in matches (chinlocks).
Don't get me wrong im not a massive Cena fan, i was happy to see Miz retain and i freely admit that the guy is cheesy and childish and if this was the attitude era he would sink like a stone. But in the same way that Rock, DX & Austin embraced the attitude era, Cena has embraced this era and i don't think he gets the credit he deserves.
I agree he can be boring, but the guy has application that some more talented people just didn't have. Wrestling as whole is not as exciting and Cena is seen as this figure-head for that and gets alot of the blame.
His appeal to me is simply what you say, everyone chants at Cena good or bad. Not sure any other wrestler can hold that claim on the consistent basis.
Just strikes me as odd that "fans" hate on him so much despite his obvious work rate for the company they watch. If you don't like the product then fine, but Cena is 1% of that problem and its a problem that includes Vince, Writers, Advisers (and i include HHH in that), Wrestlers and the lack of competition.