Lots of changes between Beta 1 and Beta 2. One of the more interesting changes I'm looking to take for a spin is that data binding can now be done to properties. Couple this with the automatic conversion between cfcs and .as value objects and you could have some pretty slick auto-wiring in your app. Getting it to actually work is the next trick.
Another change that looks like it will save a lot of confusion is that addition of a ColdFusion Flash Remoting Service project option in FlexBuilder. It takes care of managing the flex-services.xml file for you. Cool.
There are tons of other changes to the mxml language which will probably disrupt at least a piece of any previous work you've done with beta 1. Definitely check out the bottom of the linked page to see the extent of your search and replace job.
Flex:Beta 1 to Beta 2 Changes - Adobe Labs