And in fact the USA does a better job of meritocracy than most other countries.
http://www.guardian.co.uk/business/2010/mar/10/oecd-uk-worst-social-mobility
Top 3 of least social mobility (~= meritocracy): UK, Italy, US
Top 3 of most social mobility: Denmark*, Australia, Norway (* I live here)
You have been told the American Dream, I take it? Then again, "non-OECD countries" is a good approximation of "most other countries", so I guess I agree, but I would encourage you and your readers to put that statement in perspective.
Also, I suspect the opportunities for relative social mobility give people an incentive to do the things that create a rising tide of upward absolute mobility for all boats, which makes a country rich and an OECD country. (So what I'm saying: there's a selection bias due to a correlation the other way when only comparing against rich/OECD countries).
The existing structure naturally concentrates wealth up.
Personal liberty, private property and well-functioning competitive markets (with internalized externalities) seems to be a good way to create a prosperous, mobile and fair society. What could stand in the way?
For one, if you're rich, after paying taxes and life's necessities (and maybe conveniencies and amusements) you often have money by which you can earn more money, and more so than the poor.
Also, taxes and other government fiddling might be regressive. Milton Friedman points to some progressive-intended regressive transfers in Free To Choose; you can watch clips on youtube (GIYF). His negative income tax is guaranteed to be regressive.
Thirdly, you might have exclusivity deals, monopolies, externalities and government corruption/bribery/lobbying. This will tend to favor special interests (especially those too big to fail) at the expense of "the rest of us". Also, you might have heritable special priviledges.
I agree with you: progressive transfers are a good thing, especially if you're a utilitarian---grep "Daniel Kahnemann" ted.com for a presentation of why.
For that reason, many libertarian beliefs only serve to reinforce existing class structures, because so many libertarians don't understand how unfair the distribution of wealth is.
I'm not one to defend libertarians---I by and large disagree with their views---but I think you misrepresent them. Or at least you don't represent the narrow subset I know of. Which is mostly through podcasting: Brett Veinotte (School Sucks), Wes Bertrand (Complete Liberty), Stephanie Murphy and Mike something (Porc Therapy) and Stephan Molineux (Free Domain Radio).
Their viewpoint is one of deontological morals: the moral value of an action is determined by the action itself, not its eventual outcome. Furthermore, what is moral is private property and non-agression: you're free to do whatever you please as long as you leave other people and their stuff alone. This is an argument against taxation, which they like to describe as the involuntary extraction of money by force or threat of force. They tend to be market-oriented and believe that free markets don't have problems, or that their problems are better than the problems of the state.
I think the state does something wrong (see exclusivity deals above), but free markets and private property has problems in itself as well: if land is a fixed-supply resource and increasing population means increasing demand, you can speculate in land (buy now, sell later) and extract money by renting it out to a landless population. I think the fix is land value taxation: turn the value of the rent into public property. If you're inclined towards math and economics, see also the Henry George theorem.