I think the reason Cena is hated by so many of the older males is because his matches are predictable and he is the role model to so many kids. It seems to me that when something is widely popular with children, it tends to turn off adults who used to like the product before it became mainstream.
Just like every super-over wrestler WWE has ever had...once something sticks...it sticks! Why go out of the box when the box is good to you? Sometimes adult males will like a gimmick that makes kids run (Austin, Bret Hart), so I get the hatred for vice versa.
I'm not saying that Cena's character is perfect, he could use some more regular moves but it's not as horrible as some in this thread have said....and it's not changing anytime soon either.
I would be that when Cena turns heel, he will just get cheered like Austin did when he sided w/Vince...change isn't always necessary.