His promos are childish, plain and simple.
Surely not childish in a PG era? 
He's talked about "Poopy", "Snot" and "Spots" in promos. Orton might be boring, but content wise, even in PG Era he can focus about kicking you in the head. Yes I know he is more tweener than babyface, but using "PG" as an excuse for his childish promos is just weak. And if its an explantion that he is catering to kids, fine, but don't be surprised that the 18+ market get annoyed that a product once aimed at them is aimed at a different age range, and the person who stands out as being the person who is most PG, and kid like...is Cena.
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.
I don't just mean the "Hulking" up aspect. I mean one second he's down after being nailed with a devastating move, then he just stops selling takes someone out, the match is over. Its dumb, and to say, well he's a throwback, Hulk Hogan was like that is a poor excuse, people tired of that to, the business has evolved. Compare HBK/HHH selling to Cena and he's laughable. Summerslam 2010. Cena got beaten down for 5 minutes, knocked out cold (or so you'd be led to believe) on the outside. Rolled in, Gabriel misses the 450 splash, gets pinned, straight away Barrett is in the STF and tapping, Cena wins! He just suddenly sprung back to life, 100% ok. If I remember correctly wasn't HBK pissed at Cena at Wrestlemania when they fought for doing a bad job selling?
Cena has embraced this era and i don't think he gets the credit he deserves.
Maybe thats the problem.
People often knock this era, moan about the Attitude Era no more. They don't want PG. So logically they will knock the flag bearer of the era they dislike....right?
Just strikes me as odd that "fans" hate on him so much despite his obvious work rate for the company they watch.
Yeah but its kind of like on reality shows where people go. "Well he sucks, but he tries so hard, lets vote for him" And I'm like "No, if he's
****, he's
****". Same with Cena, well done for trying, but he isn't that good, I'm not going to give him bonus points for that. And like I said before, its seems society often dislike people when they appear to be trying too hard, and nobody shoves it down our throat about how hard they try like Cena does.