The gui framework used will be Reaction. MUI is there for backwards compatibility, we encourage people to use Reaction, though.
The term "good-looking GUI" is enough for some people to start a Jihad. I can tell you that the screenshots seen so far are not going to be the default look (yes, we heard the thing about the colors over and over - but you'll agree that the colors was something that could be changed in the 1.2 preferences already).
A lot of stuff was changed in Intuition under the hood, but also a lot was done to improve the looks and make it customizable. You will not be able to customize every aspect of the GUI with OS 4.0, but things will be added later with 4.1 and 4.2. However, you will be able to change a lot without going for patches - the functionality of VisualPrefs and Birdie are now part of Intuition.
Just to prevent people coming up with the rumour that intuition V50 is based on Birdie, I am talking about functionality not code. You'd think that it wouldn't be required to explicitly say something like that, but after hearing some of the rumours coursing around I feel inclined to point this out.
Having said this, I can already vividly imagine the comments about the new Intuition default look. If you ask ten people about how a good looking GUI should look like, you will get at least 12 different answers. :-D