What's wrong with Cena? quite frankly EVERYTHING. He's like Hogan circa 1991. Forced down fan's throats with constant cheap wins, long title reigns that should have been over sooner, 5 moves of doom and of course, no selling. Bret Hart will never forgive Hogan for WM IX, where Bret had a long and one of the best damn title matches of the year against Yokozuna. Only for Hogan to come down to the ring and win the belt off Yoko in 20 seconds.
Cena is pretty much like this. In all of his matches, its the same match every damn time. Cena spends about 95% of the match getting his ass kicked, then the last 5% will be Cena overcoming the odds, no selling moves and of course, winning. His promos are all the same. Pop culture references and then screaming the same point. He has zero mic skill. He's only over with 7 year olds and 20 something women. All the wrestling fans I know who like me have grown up with the New Generation of the WWF, Attitude Era and ECW, we can't stand Cena. He's everything that's wrong about it is now.
One last thing, he's a massive hypocrite. He's the face of the WWE's Be A Star Anti Bullying campaigns, yet he's openly insulted and maked gay jokes at several wrestlers including Zack Ryder.