GameSetWatch had an interesting discussion post up today about review scores and their utility for mainstream gamers. This especially applies to Wii games, for which the "mainstream" is significantly more mainstream, consisting of a lot of new gamers with totally different expectations for a good game.
Reviews tend to be written by jerks like me who have been playing games every day for 20+ years and expect not only innovation, but quality relative to that whole history of gaming. We may require a lot more out of our games: replayability, novelty, graphics, etc. More casual gamers, on the other hand, don't care about the same things we do. They just want a fun game, whether it's nice looking, short, or even licensed. Easily-understood controls and goals seem to be of utmost importance. Seemingly, so does a basis in real-world activities, like, for example, Carnival Games. So do our reviews, written as they are from our own perspectives, really serve the expanded Wii audience? And how do we tailor our impressions to their needs?
What is even more interesting to us than that aspect of the discussion is that even within "casual" games, what we see as a bad game often becomes very popular. The balancing act for reviewers is to make sure we judge casual games accurately as casual games, and try to realistically gauge whether the problems we have with them would be problems for, say, Grandma. Long-standing series are an interesting issue, as brought up by Simon Carless in regards to Bust-a-Move Bash! Is it a bad game because it fails to improve upon previous games? If so, that realistically would not matter to someone who just got their first game system.
The Wii was the highest-reviewed console of the year according to customer reviews aggregated by Reevoo. Compare that to every games-of-the-year list you've seen on any multiplatform site to get an idea of the disconnect between reviewer and fan opinions.