Interesting topic. Haven't read it all so I assume someone may have already touched on what I wanted to mention but I'm going to bring it up anyway.
Personally I tend to avoid pointing to a game and accusing it of being overrated simply because I may get my own opinions too involved where a fan of whatever game it may be could easily point out my mistakes. For example, I could list so many reasons why I hated Final Fantasy X to two of my friends and, whilst some of my perceived "reasons" are fair criticisms, they're not enough for me to call it an overrated title.
What I've found in this topic is that a lot of people simply do the same; they list the reasons why they hated the game they chose without making any strong comparisons to another title that's shadowed by its popularity or massive success, which I believe is very important to include. It's not wholly necessary in every case but you can emphasise your argument of why Title A is overrated when - look! - Title B does it far better yet received a lot of flak.
Speaking of emphasis, I find that many people make the mistake of strongly stating that a title is overrated in certain, specific respects without detailing why these are overrated features of the game. It's really annoying to read "The role playing mechanics of this game sucked" with no comparison or explanation.
Context is extremely important too. It's obviously only fair to compare a game to, say, others released within the same, close period of time - not ten years later or something as silly - but also, I feel, to titles with similar aspects / those that belong in the same genre. This obviously wouldn't be the case every time if you're just, for instance, explaining why Title A receives too much praise for its story and characters compared to Title B that was released the same week but didn't get the attention it deserved. Either way, context is certainly essential.
But I find this creates a problem when trying to call several more "unique" titles overrated as it may be the case that you find no game to readily compare it to. For example, Shadow of Colossus is a game I love and, although it surely does receive honest criticism which I can completely understand, I don't personally feel that it's an overrated title because I find that it's a difficult game to accuse of being so in the first place given simply the game that it is. You can't even compare it to Ico surprisingly because - apart from the similar approach to storytelling - they're completely different games.
Going to cut the rest of this short as I feel like I've gone on for too long but, given what I've said, I do want to point that I do feel there are certain times where a game or series of games can be argued as overrated, most particularly following the time approaching a game's release in which marketing is involved. It obviously serves to stir up hype but I also find that, through the hype, the game has already been given a pedestal on which to stand and trying to argue that it's overrated for what it was advertised as being upon its release is pretty difficult to get across to the people who bought into the hype.
But, honestly, perspective is too important for me to take yellings of "This game is overrated and here's why" all too seriously. Take the new Hitman title for instance. Been a huge fan of that series since Codename 47 and, whilst I can completely understand why - and relate to how - many fans were left disappointed in a number of ways, I also have a slightly different perspective on why the series is so great and thought that Absolution lived up to my "ideals", as it were.
As long as people can explain their arguments well then I can respectively disagree with them by countering their points with my own. You might change one another's mind, sure, but it's unlikely. People just enjoy different things about a whole host of games. Hell, they may even have a different experience in mind when looking at any game. e.g. Some people will say, "Oh, this game has the cheesiest and most awful writing" whereas someone else may find that same story fun and enjoyable for what it is.
So, yeah, with my long-winded rambling coming to an end I've lost what my point was. :P