He's a guy that sometimes does things mechanically that a first year wrestling student would do better, but I think that blinds some people to how many of the little things he does right, and to me personally, the intangibles will almost always trump how smooth a guy is in the ring. To some his selling and facial expressions may be over the top, but when you consider that a large portion of his fanbase are children, I would say they're usually pretty appropriate.
You could make a case that no "Ace" of WWF/WWE (Bruno/Hogan/Austin) has been involved in more good to great matches than Cena. It is true that Austin's career was shortened by injuries and Bruno and Hogan's primes happened in eras where top guys weren't getting the opportunity to wrestle two non-squash TV matches a week and a PPV every month, but I would say that kind of generational difference could work in Cena's favor as well. I'd argue that Cena's schedule has been as demanding, if not more so, than any of those other guys, with the modern standards for how much you have to do physically in a match being so much higher, along with the fact that by all accounts Cena is an absolute workhorse away from the ring, with all the charity work, promotional activities and side projects he does. To me, it's admirable and amazing that this guy has continued to put in an honest effort week in and week out, when he probably reached the point years ago where he could've gotten by doing less. The first match he had with Brock, post Lesnar's return, stands out to me as a guy taking a level of abuse in the ring that a lot of guys of his stature wouldn't be willing to do.
It's also worth noting that CM Punk and Daniel Bryan's best WWE matches might have been against Cena, even though some people like to say that the former two guys carried the latter.