Okay, first of all, I'm going to say any game that has enjoyed a fair amount of hype is overrated, including the post-hype of games like Psychonauts. Only a very few games live up to their reputations verbatim.
Also, I hope everyone saying "Halo" has at least completed the game they are claiming is overrated. If they have, I'll accept that their personal experience with the game was different from mine, and that's fine. They're entitled to their opinion provided they didn't lift it straight from Yahtzee.
But I should state where I am coming from with regard to the Halo series. Now, I didn't play the series until very recently, and before that I was always kind of an anti-Halo fanboy, hating the effect the games have had on FPS titles in general (turns out the shoddy wannabe Halos just never captured what made the series fun). Suffice it to say, I played Halo 1 and 2 and found them fun. And, since that's really all that matters at the end of the day to me, I've completely reversed my position on the games (not counting Halo 3, which is supposedly the worst Halo game.)
Oh, and I've really only played the Campaign modes, and I still find it wicked fun, so can everyone stop saying Halo is only decent in multiplayer? The difficulty is inconsistent and the later levels in Halo: CE are largely crap, but that doesn't detract from the amazing first 3/4 of Halo: CE. And while the difficulty curve is even more messed up in Halo 2, I found that game quite enjoyable as well, particularly all the exploration of the Covenant society/beliefs. The Halo vehicle sections have always been WELL above par for the genre, both games hold up EXTREMELY well today (particularly the AI of Halo: CE), and the weapons are all fairly balanced and useful.
Bottom line: The Halo series were NOT the first FPS games I played by far, I only recently picked up an Xbox and have never been a fan of Microsoft, and the Halo games are not the best games on the system (or the reason to own a system) by a long stretch. I have no problem with folks disliking the individual games or the series so long as they have played the games for a decent amount of time and have specific gripes or complaints. The Halo series isn't perfect by a fair margin, but I don't think any of the games can be called downright horrible or unplayable. Just my thoughts on a game series that has become somewhat of a punching bag around here.
As for what games I think are overrated, I'm gonna go ahead and say most of the newer Final Fantasy titles. FF12 in particular I really wanted to like, but it was tedious and grindy and the combat engine felt like a stripped-down, slowed down Vagrant Story or Rogue Galaxy. I didn't care for FF8 at all, and most of the post-FF7 Fantasies in general have an ascetic about them that just repulses me for some reason. The awesome steampunk FF6 art seems to have been replaced by metrosexuals swinging lightsabers on top of cathedrals (not that the gameplay necessarily reflects that.) I'm not saying that FF as a series is bad, but I just don't care for it anymore. The FF I loved in Tactics and FF6 is gone for good. At least Square had the decency to remake both games (and Chrono Trigger! Woot!).
Oh, and apologies for wall of text.