This article describes the 'Wilson score confidence interval', and describes how to use it to derive the lower bound on the nth percentile confidence interval for determining sorting criteria for thumbs-up/thumbs-down type ratings in a ratings system.

How can this be generalized to a ratings system that doesn't form a binomial distribution? Specifically, how can this be determined when each rating is a real number between 0 and 1, or when each rating is one of a set of discrete ratings (eg, 1, 2, 3, 4 or 5)?

With a normal distribution, it appears that this could be simply the average minus some number of standard deviations - but to the best of my knowledge, it's not a true normal distribution, since it's limited to the range (0, 1).