I think the biggest issue is that we are ignoring that a difference of degree is a difference in quality. Once a small number of rich people have more power than almost every one, our society is controlled in a non-representative way - an oligopoly.
Imagine if one rich person earned all the money in a country. That would be a very bad situation on many levels from concentration of power, to the induction of universal poverty. The question is, how far towards that extreme can you go while discouraging the bad effects? PG has many good points, but there was a major study that showed that US policy is mainly controlled by the preferences of the wealthy, even when the majority of the population disagrees. That doesn't sound like representative government.
http://journals.cambridge.org/action/displayAbstract?fromPag...
I don't think we should discourage the vast majority of entrepreneurs, but we should make it harder to get and maintain wealth once you move into the tens or hundreds of millions in assets or income, and past a certain number of billions, possibly blocked. I'm not sure what to do about people leaving the country once they reach that level though so that last part is probably not a good solution, but in spirit we shouldn't let a few people have too much unelected power. They're not just buying new shirts. The problem with wealth is that you can leverage the fact that people like one aspect about you (your ability to provide some business service) into another domain where they don't have to like you so much.