When you build a system where you get points for the number of people who agree with you, you are building a popularity contest for ideas.Let's compare these voting schemes to prediction markets. To function properly, prediction markets provide incentives for truth telling and for gathering knowledge. In prediction markets, even ones using play money, you have to put something at risk to have a chance at a gain, and people usually only take that risk if they think they are likely to succeed.
However, your popularity contest for ideas will not be dominated by the people with the best ideas, but the people with the most time to spend on your web site.
Votes appear to be free, like contribution is with Wikipedia, but in reality you have to register to vote, and you have to be there frequently for your votes to make much difference. So the votes aren't really free - they cost time.
If your popularity contest for ideas inherently, by its structure, favors people who waste their own time, then ... the most popular ideas will not be the best ideas ... The people who have the best ideas, and the ability to recognize them, also have better things to do and better places to be.
In most of these voting schemes, you have an unlimited number of votes. There is no incentive to do research -- it takes more time to do that than to vote -- and no cost to voting on something and being wrong.
Simple voting schemes do not create the proper incentives to get good outcomes. As Giles points out, those with knowledge have no additional incentive to participate than those without knowledge. In fact, the people with the strongest incentive to participate most likely are those seeking to manipulate the system for some external gain, a problem we see on Digg.
Please see also my previous post, "Summing collective ignorance".
[Giles post found via Dare Obasanjo]