One thing about him that has been overrated is that he's a great 'footballing' defender; that he can bring the ball out, spray it around etc. I've seen him try to give long passes many times and he's failed. This isn't a criticism of him as he's been a fantastic defender, but especially in comparisons with Vidic, some have said that Rio is a better footballer whereas Vidic isn't as good with the ball at his feet. I've actually seen Vidic give as many, if not more, accurate passes than Rio. If anything, Vidic's footballing is underrated.
Rio's strength has been his intelligence and reading of the game. And that's what makes him a top defender, even if his pace is slowly deserting him or if his 'footballing' on the ground isn't as good as some had claimed.