A couple suggestions:

When you look at a user's average rating, you should be able to see the average LEVEL rating, also. Therefore, you could see if the person is a hard or an easy reviewer. Perhaps you could divide the average reviewer's rating over the average of the levels and then get a number. The closer to 1 the "better" of a reviewer you are. Perhaps we could use this as a system to give people more karma (the closer you are to the average score in most of your reviews the more weight you get in other reviews). The only problem this would create would be that people might try to give a level the same score as previous reviewers had given it just to increase their karma.