(In reply to comment #0)
Would be handy on "busy" pages such as
http://www.mediawiki.org/wiki/Talk:Article_feedback for example.
Currently, when a discussion page has lots of comments, it is common to find links like "Show 99 replies" all over the talk page. This means that the most relevant comments in a thread (the recent ones) cannot be found through CTRL+F, unless a user expand all topics manually.
So, it would be good idea to provide users some sort of
"Expand all comments on this page"
PS: I found this bug when I was going to create a new one about this, inspired by the following comment on Village pump: