Had his career ended at 19 or 20, he'd probably be remembered in a similar vein to Duncan Edwards such was his talent and hype as a youngster. I've always thought his growing up coincided with the gradual loss of attacking adventure and perhaps naivety in Fergie's approach to the game.
As Sir Alex became more cynical and calculating in his tactics, mainly due to his frustration at not achieving more in Europe, Rooney's wild or free edge was reigned in and his wings clipped somewhat. That coupled with a number of badly timed injuries, the loss of pace and some bad advice/decision making off the field have resulted in a rather stark decline, but it hasn't been helped by this idea that he can recreate himself as a midfielder or deeper lying playmaker.
The reality is that Rooney doesn't have the positional or footballing discipline to play such a key role in modern formations. What made him great was exactly that lack of discipline or refusal to play by the rules. Expecting him to be something else now, as if because he's lost the pace and athleticism he had as a youngster, he should suddenly have developed a great spacial awareness and positional intelligence, isn't fair or worthwhile.
He was a great player for 4 or 5 years, a good player for 4 or 5 years and a fairly average one for 3 or 4 years. If he'd been good from 18-20, great from 21-26, good from 27=30 and average from 31-34, this wouldn't even be a conversation. Just because his years fell differently to most players, we shouldn't forget how bright he shone at his best.
The transfer business pissed me off, and will always leave his legacy somewhat tarnished, but he's still responsible for a few of my favourite United memories and he was a key, key part of the side that I consider the best United side I ever watched (06-08).