Slash Dementia said:
I'd add one more.
Research the game if you're going to buy it. Don't just say "oh, this looks cool," look at the game from the ground up in game-play videos, listen to the interviews with the developers (if they have interviews), Hell, even look back at the titles the company has made, see if there were any you liked. I mean, it's your money, don't just throw it around.
That's a big one. I just mentioned one of my biggest, though, in another thread: always look at the worst reviews. Not exclusively, but in addition to other reviews. If a reviewer gives a much lower rating to a game than his or her colleagues, there may be something to it. Usually, this reviewer is just pointing out things that others glossed over, but which may be significant and which you will be less disappointed about if you know of them in advance.
Another big one for me: all scores, whether they use numbers, or stars, or whatever, are to be considered bullshit. They are seldom consistent (even within the same source of reviews, highly subjective, frequently arbitrary, and tend to give an inflated view of just about everything that isn't a genuine turd. As mentioned above, the only purpose of scores is to locate reviewers who are saying bad things about a game that you might want to know. Even then, you should never attribute any meaning to the score itself.
Oh, another good one! Know the developer. Some developers (Lionhead and Bethesda come to mind) have impressive hype machines, and are talented at making reviewers believe the hype. If a game is being made by people like this, everything anybody says about their games should be taken with a grain of salt. For these companies' games, I would avoid buying at release. Let other people play the games, and see what those people have to say. You can't trust professional reviews for games from a company like this.
There are also developers which have other issues that make reviews and the like unreliable. Some companies have awful QA. GSC Game World has released two games in the S.T.A.L.K.E.R. series. Both were more or less unplayable on release, and this affected their initial reception, but I doubt many of the initial reviews and such took into account the fact that these issues could be fixed (and they largely were, as both games are much more stable now). For a company like this, you should avoid buying their games at release, and instead wait for them to be fixed up.
Large developers are also something to watch out for. Some developers are large enough that their name on a game means very little, because it can easily be on two games developed by two completely different teams. Ubisoft is an example, it has a bunch of different studios. Don't expect consistent quality from a company like that.
Basically, skepticism and research are your most powerful weapons against disappointment. Oh, and renting games. It's a lot less disappointing to find the game you rented sucks than it is to find the game you BOUGHT sucks.