Hmm ... I don't really care for the idea of leaving the "when" and "by whom" and removing all indication of exactly what happened. Yes, people will catch on eventually, but it's a bit clunky compared with the rest of the interface. For example, you could remove the word "posts" from the scrubber displayed to the right of this thread. People would be able to figure out what it means ... but why put the user in that position?
Being telegraphic in some places and not in others makes for somewhat uneven presentation.
If the length is the only problem, then perhaps you could get rid of the "Last" and just leave the word "Reply"? (As an alternative, perhaps it would be a good idea to revisit the arrow icon suggestion by wizgriz. It doesn't have to be an arrow. Perhaps we can come up with a pair of icons: one to represent the OP and one to represent replies?)
Failing that, I hope you will at least leave the "Started" and "Last reply" strings as two separate strings, so translators can add in verbs if they feel the need.
