Reply all addressed only the sender and the Cc addresses. It now also addresses everyone else the message was sent to, and leaves out the mailbox's own addresses.
A folder showed only its newest 100 messages. It now shows 100 at a time, with Next and Previous under the list.
Times in Mail now use the time zone in your Account settings. Before, the list and the open message used the server's time zone, and replies and forwards wrote the original's time in UTC.