One of the main things Obama is campaigning on is the Buffet Rule, named after Warren Buffet, who said that it's not fair how low his taxes are.
Right now, as a result of an incredibly low capital gains tax (basically the tax on income from investments instead of typical income) and various tax deductions, many rich investors pay a much lower percentage of their income in income taxes than a middle class family. Obama is proposing having people who make over $1 million yearly pay a minimum tax of 30% in order to make sure that they don't pay less than the middle class.
Honestly I don't see how anyone can argue that it's fair for the rich to pay a lower percentage of their income than the middle class (especially when we have the deficit to try to fix), so I support this.
However, the cynical side of me knows that Obama is only really pushing for this because he wants to rally the democratic party's base, which isn't very enthusiastic about his centrism.