What Cena has going for him
-GREAT gimmick. He has a gimmick which the fans of today (the target audience) can relate to. In this sense, he is similar to Hulk Hogan, Steve Austin, and The Rock. Cena's "8-mile" gimmick should be 'cool' for atleast 2-3 more years.
What Cena has going against him
Cena doesn't really have anything going against him right now, but I think it's still too early for him to be main-eventing. In recent memory, he has jobbed to the Undertaker, Kurt Angle, Brock Lesnar, and Chris Benoit. Because of this - perhaps the fans perceive him to be a mid-carder right now. If he were to be made champ right now, perhaps he would be perceived as more of a 'transitional champ'.
FORECAST
-Have John Cena stay in the US title Division. Let him have a lengthy and successful reign as US Champ. Let him beat the Rhyno's, the Sean O'Haire's, and the Hardcore Holly's of Smackdown. In half a year or so (or even longer) when Cena has CLEARLY established himself as a bona-fide winner, THEN have him move up to the main-event level.
The key however, lies within Cena getting a SOLID title reign as US Champ.
Also - what's the rush? Cena has a LONG career ahead of him. Pushing him too soon to the main-events could actually backfire.