Wow they've changed a lot. A lot of the changes come to the UI, as Skype looks completely different. For example, the whole Skype window is set out more square, and has two parts to it: your contacts sidebar and the chat pane, and all of the call stuff goes on in the right chat pane along with some store stuff.
There are other noticeable improvements too, such as much faster starting and signing in speeds (the whole cycle is about 5 seconds for me), and the whole application seems more responsive. When you call someone Skype immediately does it, you don't get a couple of seconds delay like with Skype 3.
The UI seems more consistent with the Skype website design, and looks much more awesome than any previous version did. I can't see a tab for Skype Live in sight, which in my opinion is a good thing.
You can grab the beta for Windows here and try it out for yourself.
