The difference is the one between style and substance. The Rock's promo was full of style, but Cena's was full of substance. Take away the catchphrases and the 15 year old pop culture references, what did the Rock even say? The Rock is GREAT at the insult/comic promos, but he is incapable of genuinely speaking from the heart like John Cena is. The Rock fans who actually believe that the Rock got the better of John Cena last night, and not the other way around are missing the forest for the trees. They can't focus on anything but "OMG, THE ROCK SAID STICK IT STRAIGHT UP YOUR CANDY ASS JUST LIKE HE DID 10 YEARS AGO! OMGOMGOMGOMG! OH SHIT! HE JUST MADE A POWER RANGERS JOKE! OMFG THAT WAS SOOOO FUNNY HE IS AWESOME!", and it causes them to miss the actual substance of what the Rock was saying...which was very little. Put away the nostalgia blinders. Yes, its the Rock. Yes he is hugely over. We know. We watched him the first time, ten years ago.
I don't want to bash the Rock. I love the Rock. I have been watching wrestling since the early 1980s, and he is one of the most entertaining wrestlers I have ever seen. But, in this instance, and in this ONE instance, he is in the wrong, and Cena is in the right. Cena has yet to say anything that wasn't the god's honest truth about the Rock, and all the Rock can bring to the table is insults, and while they are funny, they are non-substantive.