If you have never read a discussion or if you are logged out, clicking on a discussion on the list should bring up the top of the discussion.
However if you are logged in, the discussion will open at the last read post. This is most useful when there are new unread posts in a discussion as you will immediately see what is new. It can also be useful on discussions with no new posts as you might want to add a new reply.
An extension could be created to change the behavior. This could even be a user setting, like "Open already read discussions at the top". No such extension exists at this time as far as I'm aware.
In search results however, clicking a result should open the discussion at the first post matching the search terms.
