If it's the first time you look at 4.2, you can read about the many new features which I find more then reasonable reason to make an upgrade to this last version.
Many users will admit-probably best version ever!
Some of them might accent integration of HTML5 and CSS3 and some of them would point on core support for WEB mobile apps-a big new topic in 4.2. But all of us would admit that Responsive Web Design lead to WEB design in a true declarative way. Responsive Web Design is available also on Youtube video as well.
Cheers!