I expected it to mark everything read, but if there was a post to a thread, the thread would again be unread. It looks like only new threads will be unread.
That is what it does (or should do). What may be confusing is that Bold = Never Read. If you Mark All Read and then select the Unread filter you should see both new threads (Bold) and threads which have replies since you last read (Not Bold).
This seems a perfect opportunity to once again post my suggestion that there ought to be three states.
Your statement
" new threads (Bold) and threads which have replies since you last read (Not Bold)."
is actually incorrect because Not Bold does not mean "threads which have replies since you last read" - it means 'threads you have read and which may OR MAY NOT have had replies since you last read'.
I would like to see
Bold: new threads
Not Bold: threads which you have read and which have had no replies since you last read them.
Not Bold Italic: threads which you have read and which have had replies since you last read them.
(Instead of Italics, underline could be used - as could Bold Italic. Anything that makes a difference between threads you have read in total and threads you have read that have received new messages since you read them.
Edited byMike Walsh FINSaturday, November 8, 2008 7:32 AM"in total" added to clarify the final point