Jelly Bean isn't a huge evolution of Android like previous updates have been, but that's understandable given just how polished the OS has become. Still, as the company showed us on day 1 of I/O, things certainly have moved forward in quite a few ways (Ron provided a deeper look at some of them in his first Getting To Know Android 4.1 post). Probably the 2 biggest changes: massive optimizations and tweaks to make things very fast and smooth, and the addition of Google Now. Personally, I'm most excited about Now - a blend of Siri and HAL9000, able to intelligently answer questions, often before you even ask (or thought about asking).
The UI has also been touched up a bit, and icons and the lock screen have been tweaked. There's also more gray and less blue all around. Notifications have been revamped to show more information and allow muting them on an app-by-app basis.
Take a look through the linked posts if you missed out on the details the first time around, then let us know: are you happy with the improvements made in Jelly Bean?