Where should I start? Well, Jelly Bean will be a major update over Ice Cream Sandwich. JB fixes almost everything that Google did wrong in ICS. First of all, users will get Google Now. For those who don't know, Google Now will be a Siri-like personal assistant, the only difference is that Siri is wrong most of the times, while Google Now is right most of the times and it's a lot more useful than Siri.
The second most important feature (IMO) is Vsync timing and Triple buffering. This means that EVERYTHING will load faster in JB. And I mean everything: apps, touch interaction, website rendering, etc. By getting a smoother UI, Android will get rid of that "popular" and annoying lag. The touchscreen will no longer feel inferior to the one found on the iPhone because JB devices will respond faster.
Let's go on with improved notifications support, voice search, and camera application among others. JB devices will take far better photos, and they will understand what the users say even though they have British, Scottish, and Aussie accents. Oh, and about the notifications, users will be able to disable them and to have better control over app behavior.
I could go on until tomorrow, but you could probably get a lot more information from people who reviewed JB on tech blogs.