I personally find the jQuery UI + ThemeRoller amazing however I appreciate you’ve already looked into it . And as you probably know more about Ajax than I could hope to learn in a lifetime (me being a visuals type of guy) I accept your feedback.
However from my layman’s understanding, ExtJs seems to be very powerful and follows a typical JAVA approach with a steep learning curve, seems heavy on initial download and delays my access to the modx admin.
jQuery on the other hand seems lighter, easer to learn and with jQuery UI adding ThemeRoller offers all the widgets needed for an admin interface. As modx is open source and third party plugs should be encouraged wouldn’t an easy to learn Ajax approach open up the admin for many more community additions?
It seems that jQuery is lightweight (quick initial install) and offers all the required widgets needed for a UI.
When testing an admin UI for usability speed is a vital factor, maybe jQuery has many advantages which would make the modx theme ready, easy to customise and quick to use.
Just my thoughts