Reminds me of our education ministry which regularly says that "our students are getting smarter every year, because scores have gone up". Well, they did scale the scores. And it was possible for a score of 18 to be scaled to 50, or so my math teacher said about it then. It's just worse standards, and Metacritic sets their own weighting. I could give a game 4/10 on some blog, but Metacritic would just give my rating a low weighting.
Not to mention there's more 0 and 10 reviews on user reviews, meant to deliberately bump up or down the game's rating. I've seen someone give a 0 for a "It's a good game, but it was too similar to that other game".
But just because reviewers are getting worse, it doesn't mean games are getting better. I play more modern games than old games. I stopped playing Dungeon Keeper for DF. I don't play ADOM or Nethack, I play Dungeon Crawl. I don't play Masters of Magic, I play Fall From Heaven 2 (Civ4 mod). FIFA 2010 is significantly better than FIFA 2006. Games are getting better, assuming you ignore all those browser games and Facebook ones.
There always appears to be a dip in quality after an epic game comes out, but overall, with it's peaks and valleys, games are getting better. Metacritic took the wrong method to it, though.
But IMO, games are getting a lot more elitist and political. I've stopped reading game reviews, most of them are rubbish. Good sites like Eegra seem to have gone down and only the most populist or fanatical ones remain.