Very good post. A few other problems are the following:
1. Please note: this one does not really affect you for buying games, really only for comparing: Gamerankings does not take into account Japaneese and European reviews. This means that they have more American reviews, and almost everybody is nationalistic in some way. I'm willing to say that this messes up 90% + of the reviews. A solution is using Gamestats, as they have more Japaneese and European reviews, and less American, but I don't know how good this is.
2. Reviewers are oftenly two types of people. A) Conservative people, who do not want change. They prefer traditional fighters, racers, and pretty much everything traditional. B) People who want each game to be different from the last one. You have to change the formula to make the game good.
Problem: Neither of these look enough at the "quality" of the game. The game might get 1+/- just because of this. I haven't looed a lot at the reviews and so on, but I am certain that one group is much bigger than the other, making Gamerankings (and here also gamestats) wrong.
3. Some reviewers look at how good the AI, Grahpics, controls, character list, and so on, before fun factor, while most gamers look at fun factor first (at least I hope so). Take a game as Big Brain Academy: Wii. That game could have been made on the GBA, graphics wise. Yet, it is a fun game (alert! Biased opinion by me!), and some reviewers put that first, while others just say: the characters look WORSE than Miis! 5/10.
The biggest problem is really the review systems that have 5 things, like: Value, Graphics, controls, and so on, but no fun factor.
4. Most reviewers are REALLY hardcore. Most people, even you guys, are far from as hardcore as them. That makes a problem, because all reviewers represtent 1 part of the gaming world only. Or at least the ones that are counted to Gamerankings, and Gamestats. The reviewers in papers and so are of course not that hardcore, but this is still a very big problem.
I'll add some more when I remember them.