There are some issues with these systems which cause degradation in user feedback, here are a few:
- Not all votes carry the same weight - if an expert votes on an article and a layman does, the expert vote is the most noteworthy. There are problems with establishing who are the authorities on which topics.
- Social voting: consistently voting for people you know and your friends. These are not always votes based on the quality of the article.
- Sometimes though, users find that they appreciate articles from a certain author and track them, voting often on their submissions, but in this case, it is a genuine vote. It's hard to tell these apart.
- Some votes are generated without much thought.
- Some votes are given for fun or for profit.
There is a lot of research going on to resolve these issues, take a look at these to start with:
Complexity and Social Networks Blog of the Institute for Quantitative Social Science and the Program on Networked Governance, Harvard University
"A Few Bad Votes Too Many? Towards Robust Ranking in Social Media" (Jiang Bian, Yandong Liu, Eugene Agichtein, Hongyuan Zha)
"Dynamics of Collaborative Document Rating Systems" (Kristina Lerman)