There are some good points against such a rule in this Forbes article. Notably the inflation issue, which is a huge problem for long held cap gains. Also it was helpful for me to see some of the distinction. What it doesn't deal with, however, is the major issue that the Buffett Rule is supposed to solve, namely: It isn't fair that multi-millionaires and billionaires receiving (many) millions in cap gains per year but next to nothing else are taxed at a ridiculously low level.
I'm sure that there are better fixes available (some combination of corporate/debt tax rewriting), and it could be that the Buffett rule on its own is a bad plan, but so too is keeping things like they are.
Note: reinstatement of the estate tax to pre-Bush levels actually helps a lot, and combined with an inflation adjusted cap gains rate of 25%, reduction of the corporate rate to 20% (closing all loopholes), and probably something else...not gonna happen.