Haven't tested it yet, apart from the white balance picker which now does what it was advertised to do in the beta: Sample the whole loupe area.
I've wanted this for ages, and it makes the picker super consistent! It may seem a small improvement for many but I love it.
I also love the fact that a load of Tokina lenses now have official lens profiles. I have used a home made (by someone else) profile for my Tokina 11-16 mm, it will be interesting to try out the included profile.
Have any posters as yet tried LR4 with the upgraded preview file, and then tried deleting the catalog preview file (XXXX Previews.lrdata) and seeing if there is a speed improvement.
I'd be interested to see if using a clean previews file also helps with the speed improvement as it did for the transition from LR2 to LR3