The idea that someone is "overrated" is based on what you make comparisons with. McNabb is overrated when he's grouped with quarterbacks such as Payton Manning and Tom Brady which is often where many people's expectations lie.
When McNabb is grouped with QB's such Rivers and Breez he's not.
McNabb is a great QB who has at times shown greatness on the field. But until he wins the BIG GAME he'll never be truly great and the tendency to compare him with the likes of Brady and Manning will always lead people to consider him over rated.
I'm comparing McNabb with every QB in the NFL.
I don't think anyone has said that McNabb is better than Brady and Manning. He isn't. But in two threads some have argued that he is simply isn't very good (not true); that he is overrated (he isn't). Outside of Brady and Manning, both of whom are better than every other QB in the NFL, there isn't another QB in the NFL with McNabb's success. I don't think many talk about him as one of the top 3 QBs in the NFL over the past decade, but he is. So from that standpoint, he's actually underrated.
McNabb has had a HOF career whether he wins the "big one" or not. From that angle, he isn't much different than Jim Kelly, pre-Super Bowl winner John Elway, Marino, pre-Super Bowl winner Manning, Dan Fouts, or any other QB who never won a championship. They were all great QBs, whether they won rings or not.
Also, keep in mind what he has done with the talent around him. I'd be more critical if he had Harrison/Wayne and didn't get to the Super Bowl. That's why I think what Brady did is so great. He won his Super Bowls without having the best WRs in the league. When they gave him Moss, Welker, etc., he set records and went to the Super Bowl. McNabb did the same thing (achieved with average or even sub par talent and went to the Super Bowl when they got him T.O.).