You can display messages in the way you desire (single line per message, showing From: , Subject:, Date: ). And you can do so both in Message view and in Conversation view.
However, in order to do so, you need to turn OFF the right-hand reading pane. You can instead use the bottom reading pane, or turn the reading pane off entirely. (To do so, select 'view' from the menu bar.)
You can also right-click the column headers (tag, subject etc...) and choose which columns to display.
With the right-hand reading pane ON, you can't change the columns shown, nor the order - because of the compactness needed.
I agree that the current view is not ideal.
If you wish to change or offer options with the right reading pane on, you should file a new bug, and vote for it.
You can do this at
Bugzilla Main Page - Zimbra and put a link to it here so other users can vote for it, or comment on it. (Search first to check there isn't already a bug for this!)