Here's some excerpt from their release page:
[...] Many of the most popular and commonly used jQuery methods have seen a significant rewrite in jQuery 1.4. When analyzing the code base we found that we were able to make some significant performance gains by comparing jQuery against itself: Seeing how many internal function calls were being made and to work to reduce the complexity of the code base. [...]
[...] In jQuery 1.4 we’ve fixed 207 bugs (compared to 97 bugs in the 1.3 release). Additionally we’ve increased our test coverage from 1504 tests in jQuery 1.3.2 to 3060 tests in jQuery 1.4. The jQuery test suite is 100% passing in all the major browsers (Safari 3.2, Safari 4, Firefox 2, Firefox 3, Firefox 3.5, IE 6, IE 7, IE 8, Opera 10.10, and Chrome). [...]
So I'm really thrilled to try it out. The only drawback is that they still support IE6. I've been told by a jQuery guy back in Toronto in Stack Overflow that they would phase out IE6 in 1.4 to significantly reduce the core size. Unfortunately it didn't happened.
But still, I have some plans to revamp the home page of PokerPlanner.org this week-end, I'll include jQuery 1.4 in my todo list for sure!