There were definitely some big changes for the 3rd edition. We added a chapter on Ajax, the browsers and browser versions covered were dramatically updated, and Paul really reworked a lot of the guts of the example throughout the book.
The 2nd edition is still a good book and you'll learn from it, but the 3rd edition is definitely more up to date with modern practices.
Wiley Technology Publishing
Wrox online library: http://wrox.books24x7.com