Trees are supported - see references represented as trees in the User Guide.
Animation/Effects - it depends what you mean here. Some animation and effects are definitely supported.
Drag and drop - not supported currently.
As far as UI enhancements is concerned - Aware IM exposes an XML interface to the web server. Using this interface you can create your own custom UI based on anything you like - you don't even need to use EXT.
The implementation of the current Aware IM UI based on the XML interface is available almost "open source" - it is available as a set of JavaScript files located in the AwareIM/Tomcat/webapps/AwareIM/aware_ext folder.
If you are a programmer who can write AJAX-based Javascript based on the XML interface exposed by the server you can modify this standard interface as you see fit - you can certainly enhance it using jQuery or anything else you like. You can even share your enhancements with us 😉
