Well, I was going through their list of changes and I found an interesting one:

Throw an error on $(“#”) rather than returning 0-length collection (80022c8)

I still remember that this problem had almost given me a heart attack in my college project days.

The fact that they have re-written jQuery's legacy functions, show() and hide(), excite me to use the new jQuery 3. These methods were a headache for inline elements. There are obviously many breaking changes, so upgrading to jQuery 3 won't be very easy.

I also appreciate that functions like width() and height() will return decimal values instead of rounded integers.

The most important improvement is the use of requestAnimationFrame by default for all jQuery Animations. This will make a huge difference while animating using new jQuery.