The basic theory is that by heavily taxing productive people to fuel a socialized state you 1. create disincentives for hard work and investment (why should I bust my ass to make a dollar more if you're going to take most of it away from me?), even driving some wealthy people to leave the country, and 2. further inhibit economic growth by spending money less efficiently than private individuals and industry do (because of bureaucratic overhead, etc.), which creates waste and corruption and slows economic growth and increases unemployment.
Rich people do one of three things with their money: They can spend it (which creates jobs and fuels economic growth), they can invest it (which creates jobs and fuels economic growth), or they can give it away to charity (which helps the poor and public far more efficiently and responsibly than government programs do.) Bill Gates, son-of-a-bitch that he is, is giving away virtually all of his vast fortune to charity. That's the sort of thing the super-rich do; this notion that government is helping society by robbing the rich to give to the poor (after taking a fat cut for themselves) is a bit of a delusion of the proletariat.
Does that really matter? Well, in 2005, the UK had a per-capita income of about $30,000...compared to the US's per-capita income of almost $42,000. The UK's economy grew at 1.8%...while the US's economy grew by 3.5%. (Source:
CIA Factbook.) Real income (adjusted for inflation) has almost tripled in the US in the past fifty years. The reason is simple: Innovation, risk-taking, and investment are rewarded here; success is not seen simply as an excuse for the government to take away (and misspend) money that wealth-producing people 'don't really need'.
Socialism trades the promise of tomorrow for the security of today. If that's what floats your boat (if not your economy), have fun; America will wave and smile as we go by in our yachts.

I would say 'fuck the socialists', but really, I don't need to; they fuck themselves.