I really do agree with Sirean, but at the same time I can see why so many excellent games like Bioshock and GTA4 face so much backlash. The problem exists because these games are built by the media to be something they obviously aren't.
This industry's level of journalism is absolutely piss poor, and has reached an all time low since print media began its spiral downwards. Due to the limited nature of print, writers made their articles concise, with each sentence given careful consideration. Writers were also qualified; most of these guys had college degrees, if not in writing, then in at least something.
While it isn't easy to get a writing job at a games website, it is still a lot easier than it was to get a job at a magazine. I used to read a lot of gaming mags like PC GAMER, CGW, EGM (at times), PC GAMER UK, and never remember those magazines bestowing a level of praise on games that bordered on fellatio, as is the norm. today.
It seems that the best game ever made seems to be released every few months. Games like Bioshock, MGS4, GTA4 were all praised like they were gifts from the gods. It is only natural for the average fan to be a little bit disappointed when expectations are raised to such a level.
Part of the problem is that all of these writers speak in extremities. Either they think the game SUCKS, or it is AWESOME.
The other issue is that the industry is far more corrupt than it was. Nowadays, it is very common for game publishers to wine and dine reviewers. Unfortunately, they choose the writer from the staff of a website, that will get the exclusive review. It is why you always see those doing exclusive reviews, scoring games far higher than the industry standard. It is normally because these people are normally huge fans of the genre, and are picked by the publishers for this very reason.
Unlike movies, there is no group advanced screening for a bunch of critics, but rather, for a couple of carefully selected ones.
When GTA4 came out, there were tons of game critics from sites like 1UP that were featured on news channels like FOX, and were praising GTA4 like it was the greatest thing since sliced bread.