Fans are really getting sick of vanilla good guys. I honestly think it's why Punk worked so well. He was a good guy that had an edge. Can't say that about a lot of faces any more.
No, they're just sick of the vanilla good guys that happen to have no in-ring personality, either.
Bret hart was about as vanilla as it could get in the early 90s, but he had a gimmick to him:
#1: He was a master technician, the excellence of execution. Every match he had a strategy and he put it in motion. Even if it involved heel in-ring tactics at times.
#2: He always gave it his all. No, really, when guys like cena say they DO. NOT. QUIT, Bret said that and the matches were booked to back that up. He would always find a way to counter a submission, claw his way to the ropes, or stay in a submission for like 10 minutes until someone threw in the towel (see: Bret vs Backlund).
All you need is SOMETHING to make a guy stand out. Cena is just as vanilla as it gets.