as tails said, the ratings are for all registered users, and are permanent (though of course users are free to change their rating at any time).
as the system is fairly new, i wouldn't get too concerned when a problem has only two ratings and you disagree with the average. let's see how the ratings balance out after some more time.
the Stats page shouldn't just show the problems with the highest average as Top Rated, but should do something like a confidence interval:http://www.evanmiller.org/how-not-to-so ... ating.html
this is my fault since i always had it as an average.
excluding a subset of submissions from a rating system is probably not going to happen. if something gets submitted to digg or StackOverflow or any other social site i can think of, it suffers the same trial by fire as any other submission.