There are two types of overrated: touted as good but actually ordinary, and touted as brilliant but actually just good. Riewoldt is a great player, but is nowhere near as good as Bruce McAvaney tries to tell us he is - and that makes him overrated by definition.
I also disagree about Sam Mitchell; I think he is as good as they all say.
Dale Thomas, for me, is the most overrated player in the league. Quite simply, he does the wrong things. He's flashy, and he looks like a genius when things go right, but he does low percentage things consistently, and he would drive me mad if he were in my team.
I think Matthew Scarlett's defensive skills are overrated. What he adds to the team as a rebounder makes him a deadly player and well worth his accolades, but he does not have the best traditional defensive skills in the game today, and certainly doesn't warrant comparison with the greats of yesteryear in that respect.