Cena is smart. Work hard, keep your head down, when the boss says jump, ask "how high, sir?" and you'll get rewarded.
Cena's problem relative to Lesnar is that Cena has no edge. He's the epitome of a corporate 'yes man'. The only thing that worries me more than Cena letting WWE dictate who he should be is Cena actually deciding that this quasi-military-salute-the-kids-wear-the-merch-jean-shorts-and-wrestling-gear-even-when-you're-fired-rapadoo persona is who he wants to be. That scares the shit out of me.
Lesnar is a big dumb oaf, as massively uneducated as he is massively...massive. But he's a big dumb oaf with (big, dumb) convictions. He treated WWE like a stepping stone, thanklessly walking away from pro wrestling and using his name and notoriety to team tryouts for the NFL (you think he'd get anywhere if he was "Brock Lesnar, part time stock boy at Tractor Supply Co."?) and then on to greater legitimacy and success in the UFC.
Now, Lesnar capitulated and proved himself a company man as well in UFC, when he trashed Bud Light after a title win and later, at the press conference, made a very contrite and humble apology to Bud Light (even had one during the presser, if memory serves). But that was a Lesnar slip-up and he still conveys himself as the loose cannon.
Cena is loyal to the bitter end. He has no edge, no urgency, no sense of sincerity. He believes in the corporate myth. That's why you'll never see him in TNA, or ROH, or anywhere other than WWE. Hogan wasn't concerned with putting WWF over, he was concerned with putting Hogan over. He did more harm than good to WCW and now TNA as well. So, if I'm WWE as a corporation, I want a big capitulating baby like Cena, who believes in the corporate "mission statement" or whatever. He's the guy who believes the great big lie.
But that means the product is centered around a person who has no soul to convey to the crowd.