There are some big advantages to self-publishing that Tim left out.
1. You can write the book you want. No need to try to fit into a publisher's product line or marketing plan. The worst and most demoralizing thing an author can do is write someone else's book instead of his own.
2. Creative control. You aren't subject to the whims of a quirky editor who has weird ideas about what the market wants (usually as a result of sitting around with lots of marketroids).
3. No need to grovel. You don't need to put huge amounts of effort into selling the book, or the idea of the book, to a publisher.
4. No need to negotiate contracts. Standard literary contracts are notorious for being the most obfuscated, author-hostile, nasty, full-of-gotchas-and-pitfalls contracts in the world (second only to, perhaps, recording contracts).
5. You can keep your work fresh. You can revise when you want to -- or when your readers tell you to -- rather than when your publisher runs out of inventory.
6. You can change course if an opportunity presents itself. Rather than writing to an outline which you painstakingly negotiated with a publisher, you can "turn on a dime" if new technology or new challenges arise.
The bigger the publisher you're thinking of dealing with, the more these advantages can matter to you as an author.