Right now my favorite improvements are the refined Develop settings and the new Book module. The ability to layout and order books right from Lightroom makes life much easier. (no more switching to web interfaces, or third-party software.) Right now it only works with Blurb, but you can export your book to a PDF file to send to the book-maker of your choice.
Since LR4 has updated Develop settings, a lot of us photogs have been going back and re-editing some of our more problematic (i.e. sucky) images. For me, a great example is the one above. I realize some of you like this image, but for me, it has never been "good enough", via the editing in LR3. Since LR4 has revamped the whole Basic part of the Develop module, the ability to tweak the Highlights and Shadows locally now exists without affecting the overall brightness of the image, without using a mask!
LR 3 edit:
And now the LR4 version:
(Yes, I realize the cropping is slightly different!)
But as you can see, much better noise reduction, sharpening and the ability to control the brightness of the light trails, without affecting the overall image.
Give it a try:
(The link takes you to the NAPP site that has videos about each of the features and a link to Adobe's download site under "Get It Now")