I don't think five-star ratings are a natural choice for expressing opinions about political ideas. There are a number of reasons for this:

• With a political idea you want to know whether people agree, disagree, or don't care... not how much they like it on a 5-point scale.
• I suspect that people who are familiar with the principles of averaged 5-star ratings will tend to always vote 1 or 5 anyway... these are the ratings which will have the biggest impact on the average, and if people make ratings its because they want their opinion to be heard.
•  An aggregate score produced by subtracting "down-votes" from "up-votes" would probably be more useful for determining what the "popular" ideas at any given point are. An aggregate score gives you a measure which combines the number of people who are interested in the topic with the proportion that agree disagree - in one number. Compare this to average 5-star ratings: an idea which 10 people have rated a "5" has a higher average than an idea which 100 people have rated "5" and a single person has rated "1". 

I see another idea here about having a minimum number of votes for an idea to appear on the "highest rated" pages... this is an alternative way to address the same problem.

An up/down voting score captures both level of interest and level of agreement, an averaged 5-star rating can only ever capture the level of agreement.