Why should someone making $40k a year pay a higher percentage of taxes than a billionaire?
It's part social engineering, part pandering to the rich.
Capital gains taxes are 15% only if you hold for over a year. If you hold for less than a year, you'll be paying 35% or whatever. The intent is to stabilize the market (fewer trades in and out just to capture fleeting price changes), and to protect the amateur investor from herself. Yes, we can bicker endlessly as to whether it has that effect, and whether such nannyism is warranted, but that's the reason. In general, I support such a scheme, though we could bicker about the flat nature (i.e. why not 25%, say, once you top, say $5MM in gains).
It does not just benefit the uber-wealthy - any person with an investment account benefits. Middle class and up.
On the flip side, I think a lot of middle class are actually getting clobbered a bit due to this law. Most middle class by mutual funds, not individual stocks. Mutual funds (except for a holy few that I'd actually recommend putting money into) live and die by quarterly results, and are endlessly flipping stocks to get their numbers better for the short term. The upshot is a heavy tax burden on the fund holder (with a few exceptions, never, ever, put your money in a mutual fund, unless you like losing money for the ego and job security of a manager)
Books have been written about this - a few posts on JREF aren't going to solve the problem, no matter how much we pat ourselves on the back for being 'skeptical'. The upshot is that the rates are designed to encourage long term investment into America by encouraging people to own and hold businesses. An admittedly unfortunate side effect of that is the truly low taxes somebody like Buffett ends up paying.
As for Buffett writing an extra check to the IRS - he has been directly asked that question on CNBC (I forget by who, probably Becky Quick). His response was that he was entirely willing to do that, if that was the best choice, but that he feels that he is somewhat better at asset allocation to solve social problems than the US Government is. It would be easy to cynically dismiss that, except that he gives away billions every year. He walks the talk.
