I was largely agnostic when it came to the Bush-era tax cuts. But I had no idea how much taxing the rich taxed small business.
From what I gathered (via this video), all revenue from a privately held company (which small businesses are) counts as income for the owner. If the efforts of dozens of people bring in half a million, that's the legally the same as a CEO of a big company making half a million. In the former case, that money goes to employ the people to keep the business afloat. In the latter case, it's not. The narrator in the video underlines how risky it was for him to hire more people since it's unclear if the tax cuts will expire or not.
The video's produced by the Small Business & Entrepreneurship Council, so I assume they're correct that the tax system works as described, but of course there could be many exceptions that exclude most small businesses and the narrator's just in a bad place. Still, I think when most of us think "the rich's income" we're not thinking small business revenue.
That's not to say that small business is the "key" to economic recovery...I don't think it is. But it's not insignificant, either, and allowing the cuts to expire look less and less like a good idea.