When rolling the pointer over the message dividing line four new instant selections appear for trash, reply, reply all and forward.
If you roll the pointer over the message dividing line four instant selections popup for Trash, Reply, Reply All and Forward.
This can be somewhat useful and avoids a trip to the tool bar or menus, especially if you are in a threaded message with numerous responses.
I never noticed this behavior before for the Mail client, and thought it was worth a mention.
[crarko adds: I looked in Snow Leopard and couldn't reproduce, so I assume this is indeed new behavior in Mail 5.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2011081709453290