Regarding ad_form and the list builder, did you base this work on .LRN 2.3.0 or a previous version? Nima already did a bunch of work in this area that got into .LRN 2.3.0 ...
I would tend to prefer this to go to HEAD, as 2.3.1 is supposed to be a minimal upgrade with minimum testing required, if you've changed many files.
I don't know if anyone has the time available to do accessibility and validation testing if you've done much changing. In theory, of course, switching to the list builder and ad_form helps a lot, but the list builder in particular allows for embedded HTML in the Tcl code that builds the list so there's ample room for mistakes to creep in ...
And I know folks would like a 2.3.1 release ASAP, too.
I don't have strong feelings about the forums stuff. Apparently people mostly use the threaded forums, because the non-threaded ones for all practical purposes simply didn't work ... so there's probably no reason to rush this into 2.3.1.