What's the difference between the Do Not Track behavior being proposed for IE 10 and simply deleting all cookies from the browser when you exit?
Huge difference. Do not track is an initiative that's basically a compromise between browser makers, privacy advocates, and advertisers. Advertisers that respect the do-not-track preferences won't use behavior tracking - won't stop the advertising, but it cuts the creepiness a bit. While implementing DNT is currently voluntary, for advertisers that respect it, this should stop all tracking.
Deleting cookies potentially creates significant inconveniences including being able to stay logged in to websites, and thanks to major social networks and alternative methods of tracking like the so called "super-cookie", isn't entirely effective, but it will still help.