This morning I tried expanding the list of comments of an answer, but it doesn't work.
I tried also on Stack Overflow, on How to quickly theme a view? and I was able to expand the list of the comments for that question. It seems it is a issue on EL&U.
EL&U and SO report to be using different revisions of the software; while Stack Overflow uses the 2011.4.4.1 revision, EL&U uses revision 2011.4.4.4.
Still, when I tried on programmers.SE (e.g. on What should a developer know before building a public web site?), I was able to expand the list of comments. It seems the problem is specific to EL&U.
I am using Firefox 4.