EXT JS was amazing. It allowed one to, in a very, very short time, create highly capable tables of data with a lot of tasty controls and features that just came straight from the framework. It was very, very complicated to learn, but the guys that did so (I was a novice), were able to crank up some serious business. It is my understanding that YUI was a precursor to EXT JS and is striving toward much the same goal.
Listening to the YUI3 people doing a presentation to some sort of user group, I was impressed by their intention to make YUI work in a way that is very consistent with the basic ideas of the DOM. Since this is the foundation of every web browser presentation layer and, thus, every application, that seems like a really good thing. Further, the main thing I thought about EXT JS was that it felt like none of my previous knowledge really helped me master it.
The other thing they talked about, that I had forgotten about and that I liked very, very much, was the ability to create custom events. It's nice to have "onclick", etc, but sometimes I want to have an "onStupidUser" event that allows me to do something outside the normal flow. I used this a lot in my work with EXT JS. The YUI guys talked about some good improvements upon the theme.
But, it's a series of decisions for another day.