Why is it for such a short period? It seems like this kind of thing should be for a full year after the law is passed or something similar.
You really should read the text of the actual bill, which starts addressing tax matters on page 51. Alternatively, have your accountant or tax lawyer explain it for you. It modifies the existing rules, which are already complex and require knowledge (or diligent research) of the tax code.
Edit: OK, I suppose it's not that obvious. but if you keep reading, you'd see the next part of the law adds a 5 year carryback for certain small business investments, which is basically a retroactive tax credit allowing some small business investors/owners to get a refund of some taxes paid over the last 5 years. Like, say, capital gains from investment in a small business.
That's not a full explanation (or even a meaningful one), just an example of how different provisions can interact to give results that are no obvious when they're taken in isolation. You should read the whole tax section, there's a lot of interesting stuff in it. Reading bits of laws in isolation from their context often gives a misleading or incomplete picture.
Here's some accessible and more detailed information provided by an accounting firm that provides a better overview of ways this law might benefit startups and self-employed hackers: http://www.mohlernixon.com/informed/2010/09/focusing-on-the-...
For going concerns with profits, sure, the other stuff matters (although I think the more liberalized depreciation, and allowing cellphones to be more easily expensed, is the relevant part). Anyone with enough revenue and profits to care probably has an accountant who is going to handle all of this for him, so it isn't as individual decision centric as the capital gains issues.
The idea is to create a burst of activity this year. It would then be fully attributable to the current congress.