Breaking Mauro Rosales' game into component skills is pretty easy. There are a few things that he does amazingly well and a few that he's pretty bad at doing. He's quite MLS-y in this as he is a specialist. Some of those skills will not go away with age.
Most notably he serves one of the best long balls to head height this league has seen. Yes, that good. He's up there with that Beckham dude and Brad Davis. He's still that good. While his numbers were down a bit, Seattle Sounders FC only had one good header of the ball this year (EJ). Mau can serve it on the run in traffic or on dead balls.
Another thing he can do from dead balls, is this
It's not going to happen often, but just being capable of doing that is special. His short passing is good, but for some reason does not work centrally as well as you think it should. Seattle played him as a withdrawn forward a couple times, but he's a right winger. Not a right mid, he's too high up and not good enough defensively for that label.
In fact, that's his biggest weakness. He is not good on defense. Sometimes he'll try really hard, but he's just not that good at it.
His other weakness is that Rosales is fragile. He doesn't take hits well, goes down easy and carries injuries a bit long. As an older player Mauro is losing some of his speed.