Consider integrating jQuery and using jQuery effects, like slide effects. Assimilate all the UI wonders that have developed, rather than trying to reimplement them ourselves.

I'm still holding back on this one, just because things change so fast while our conservative status quo is fine. Users have the option of adding jQuery stuff on top of Composr very easily, and even some of our addons do.

It may be within a few years we are all using something like Dart, or a Javascript 3 that incorporates similar stuff to jQuery. If that's likely, I'd rather not go ripping out long functional parts of Composr, just to replace them with something temporary that is equally messy (jQuery ain't any prettier).

Regarding getting rid of YUI, go for HTML5 webform shims as much as possible:
http://www.useragentman.com/blog/2010/07/27/creating-cross-browser-html5-forms-now-using-modernizr-webforms2-and-html5widgets-2/

select2 integrated for standard list fields
jQuery and jQuery UI were added to core
YUI was dropped
HTML5 date&color input widgets, with jQuery/jQuery-UI fallbacks, were implemented
JAVASCRIPT_MORE was dropped

Not done:
- meta keyword input (would just create more problems, easier to just type in a line than fiddle with entering custom selections into a list many times)
- author/username - we already did an alternative jQuery plugin for this
- cog icons - some users already struggle to find topic options, best to leave them visible by default
- jQuery effects -- we are using a lot of CSS effects in v9 and hence v10 too, jQuery was of doing things are getting less common and are slower -- however with jQuery in core people can add things in using normal themeing very easily
- replacing much core Composr JS with jQuery -- I don't want to add jQuery as a permanent dependency for basic things, and I want to be able to continue to really streamline/optimise/customise our basic JS library features -- however now that jQuery is core, any screen may call up and use jQuery code as it wants to