He is horrible on the mic. Never has been good and never will be good. The rapper, thug, bully promos just don't work well, especially when they are directed at legends such as HBK and HHH.
And I agree with legolas... it is very hard to screw up when you only have 5 or so moves to do. Even khali seems to do his 3 moves fairly well. That doesn't make either good in the ring though. Add on to this that Cena just cannot sell moves it is ridiculous. He tries to act like he is a 7 foot 3 to 500 pound Nash, Big Show, Kane, Undertaker (etc) and does not sell moves. Those guys could get away with it, Cena cannot.
He also has some of the worst facial expressions in the business today. Nothing is natural with this man