So, for the last 106 years, John Cena has been the main draw of the WWE, playing the same clean-cut, babyface, square-jawed superhero who crushes all opposition and gets the crowd behind him.
And for the last 103 years, a certain segment of WWE audiences has been tired of it. For a while now, he's been getting booed and receives negative chants.
But in 2011, the WWE began acknowledging this, selling merchandise based on the "Cena Sucks" chants, having WWE legends like Rowdy Roddy Piper call him out, having Cena cut promo after promo embracing the audience's hate and even developing a drawn-out, main-event level non-title feud with Kane that teased a Cena heel turn and dragged in Zack Ryder and Eve.
Now, Cena is still the top draw, the face of the brand, the ambassador, the main merch-seller in the WWE. CM Punk is handling the RAW main-event duties well and Ryder is selling lots of merchandise, but Cena is still the man in a number of ways.
To some, Randy Orton has become (since his turn) the main babyface on WWE programming, but he doesn't do what Cena does: He's an animal, a vicious attack dog, a coiled spring. He was best known as a heel, the Legend Killer, a man who hears voices. He destroys his opponents, not because he's supposed to or because they've wronged him or another, but because that's who he is.
Likewise, CM Punk is a hero because he's sticking it to the man, because he's an individual, because he drops pipe bombs and is generally a crabby person. He's not clean-cut, he's not a soldier or a big brother or a friendly face. He's a sarcastic and sometimes bitter man who happens to be the "best in the world." He's a personality. And he's not a Boy Scout, he's a malcontent. Plus, good as he is, he's skinny. He's not the dominating physical presence that Cena is.
The rest of Cena's physical/weight class is full of heels: Wade Barrett, Jack Swagger, Jinder Mahal, and until recently, Sheamus.
Sheamus actually flirted with facehood (or at least, less-strong heelship) in 2010 when he ran out to help John Cena protect himself against the six-man Nexus. The two brawny brawlers used two chairs and the protection of the ring ropes to keep the six rookies at bay. Of course, Sheamus would face Cena soon after, but it established Sheamus as a man who liked a fair fight.
He also had a bit of a brush with comedy in late 2010 when, while still booked as a dangerous, hair-trigger ball of anger, he participated in the famous "Tea Party" with Santino, where he took the brunt of several "inadvertent" ribs from Santino, and Kozlov reminded him Santino beat him in "the biggest upset in double-double E history."
Then in July of 2011, the brash Sheamus volunteered to face Mark Henry at a time when no one else would (and most of those who did would be off TV the rest of the year and come back changed men).
This began Sheamus' true face turn, and it would continue when the majority of WWE employees walked out of RAW in a vote of no confidence in Triple H, due in large part to the dangerous actions of Mark Henry and Awesome Truth. (So... why doesn't the same thing happen because of Kane? Oh right... WWE.) Best of all, he lost to Henry. Henry's Hall of Pain heel run was just getting solid, and a loss would have taken the air out of it. Sheamus looked respectable for his attitude and ability, but was able to take a loss where Cena might not have.
And then in October, Sheamus was one of only three men, the others being Cena and Punk, to stay on. Sheamus said he respected Triple H for dealing with him man-to-man rather than using legal resources, back when a Heel Sheamus attacked an unsuspecting Triple H with a lead pipe, Clue-style.
This solidified Sheamus as a true babyface (Punk referred to the other 3 men as "smiley good guys"), and he further developed his persona as the year went on. He blended his smiling, "what's the crack, fella?" attitude and his Irish folktales and family stories with his previous angry bulldozer-style aggression into a sort of Jeckyl/Hyde persona where he is happy-go-lucky until pushed to his breaking point, where he goes over the edge into a violent rage.
WWE even began running videos that established this "spirited" attitude while commentators wondered openly if he was a "hothead" or had a "mean streak."
Giving Sheamus a guaranteed title match at Wrestlemania is a big step, but it makes a lot of sense.
Sheamus already had two WWE title runs, but they weren't very successful as he was very new, and despite (or because of) having a huge push early, people weren't keen on seeing the inexperienced heel carry gold.
Sheamus has now been around a while and established himself as a high-quality wrestler and a popular personality, so a title run makes sense both in terms of kayfabe and in being over with the crowds.
I've kind of gotten away from my point, which is this: Sheamus is what John Cena should be, but better.
He's a big, strong, agile combatant who fights with intensity when called for, is a smiling, happy-go-lucky guy with an approachable face, a lighthearted attitude, a willingness to be part of (or the butt of) a joke and he's a big part of the "Be a S.T.A.R." thing they've got going on, yet he has an honest-to-dog mean streak built into his character. He doesn't have to be pushed week after week or see someone get victimized in order to tap into his hate—it's right there. But as soon as the job's done, he'll be smiling again.
He's not the half-psycho that Randy Orton is, he's not the cold monster that Mark Henry is and he's not the shiny Boy Scout that Cena is.
He's a little of all these, but mostly he's a fun, respectable guy who will kick the crap out of a guy as soon as look at him, but doesn't cheat or whine or talk too big about himself.
Cena wishes he had that mean streak. Or rather, we wish he did.