I like the fact that the reputation score gives a sense of achievement, but it's far from perfect. 
I liked the UA project by @scipio a few years ago. It was using an Algo similar to Google PageRank, based on who was following whom. The problem was that you need to choose how to initialize the reputation. They chose to do it by giving a strongest initial wheight to top witnesses and as a result, the reputation was quite biased toward witnesses. 
Conclusion : it's really hard to make a good and fair reputation system ^^