Here's a great video from the Cato Institute giving just 5 reasons why taxing the rich is a crap idea.
The video uses an argument I've employed against my left wing, economically illiterate (I know, I know, it's tautologous) mates. They want to use taxation to reduce a person's consumption of alcohol or cigarettes but then aren't able to make the connection that taxing productivity will have exactly the same effect - there's less of it.
The economy destroying emissions trading scheme currently being debated in Australia works on the same principle. Use the tax system (or a credit system, which is the same thing) to reduce output of CO2.
More tax. Less whatever.
Tax productivity. Less productivity.
It's not too hard to understand, surely?