Many of these are improvements to the smoothness of rendering on the device - it even briefly boosts the CPU when the screen's turned on to make things super slick and speedy. Google dubs these little performance and software improvements as being "buttery", assumedly referring to some nice spreadable Lurpak rather than a solidified hunk of congealed milk.
The other big addition here is Google Now. This adds personalised recommendations and information based on your browsing history. Some people love its perceptive usefulness, others hate its pervy, intrusive nature.
One thing's for sure though: the Siri-like voice search feature is awesome.
As ever, these tips are based on the vanilla installation of Android, so your mileage may vary depending on how many layers of extra "features" have been added by your phone or tablet's manufacturer.