|California Coastal Sunset - M9, 28 Elmarit, ~f/8, 1/60s, ISO 160|
The feature that I think will have the most impact across all types of photographers are the reworked tonal controls part of "Process Version 2012". It took some getting used to, I had to change the way I worked with my images but the end result is a level of tonal control that I can't get in Lightroom 3.
In Lightroom 3 I would set the white point with 'exposure', the black point with 'blacks' and the tonal distribution between your white and black points with 'brightness'. I usually ignore 'fill light' and 'recovery' as I find 'recovery' reduced highlight contrast and I didn't like the global nature of 'fill light'.
In Lightroom 4, I instead use 'exposure' to get the bulk of my midtones where I want them. Then use 'shadows' and 'blacks' to both control shadow contrast and to push the shadow tones around and use 'whites' and 'highlights' to do the same with the highlight tones. Since these new parameters can go negative and positive, I find it easy to get my tones where I want them and maintain shadow and highlight contrast.
The other cool thing about the way these parameters work in Lightroom 4 is that they are based on the tonal distribution of the image you are working on. In Lightroom 3, it was possible in scenes with extremely low contrast to push the blacks slider all the way to the right and still not have enough contrast. This shouldn't be as much of a problem in Lightroom 4. These same smarts are also carried to the 'clarity' control.
The best part about all of this is that the 'shadows' and 'highlights' adjustments can now be done locally. This is really powerful because it means that I can lift the shadow tones in a part of the image or pull back the highlights locally. For an image like the one above, it takes only seconds to get all my tones where I want them without going into Photoshop.
Oh yea, and the last nifty thing about the tonal controls is that highlight recovery has been reworked so that you don't get those hue shifts that you do in previous versions.
There's a lot of neat stuff to explore in Lightroom 4, so definitely check it out.
EDIT: I forgot to mention this originally, don't change all your existing images to Process Version 2012. They will look very different and you'll have to process them again. In fact, since this is a beta, you shouldn't use it with your production catalog either.