The problem is that gifts to charity aren't coercive. I know, it's strange to say that it's a
problem if it isn't coercive, and I'd agree that many of the inefficiencies of government stem from the coercive nature of taxes (no need to convince people to give you money if you can just take it by force). But I'm really a moderate in all of this - I personally don't think a modern state can survive on donations, or a "pay only if you think the government will do a good job with the money" tax code. We're gonna have to collect some taxes, rich people (in my opinion) should pay more than poor people, and there's no way people, rich or poor, are just going to pony up out of a deep affection and fondness for the federal government.
If tax rates are going to be paid and progressive (they actually aren't), then they'll have to be coercive. I think what Mr. Buffet is arguing here is that the rich need to be coerced into paying more in taxes than they currently do. I'm inclined to agree with him, even if I have to hold my nose as I do.