You don't lose the capability to use Find & Replace, however. Whenever you do an inline find while composing a message, a Replace checkbox appears; check that box, and you can enter your replacement text, with options to replace just the current instance or all instances of the search string. The location of the find/replace fields within the current message window is more convenient than--and reduces the screen clutter due to--a separate Find window. It's also less confusing, as you'll never see a Replace option for an incoming message. On the other hand, you lose the Lion option to perform case-sensitive searches, but we suspect this won't be an issue for most users.
Notifications in rules and selective notifications
(Image Caption: Mail's Notifications options)
One of the marquee features of Mountain Lion is an iOS-inspired Notification Center that centralizes alerts and notifications from all your Notification Center-enabled apps. Mail takes advantage of Notification Center to alert you to new messages, but instead of displaying a notification for every new message, as with iOS, OS X Mail lets you choose which new messages should trigger notification alerts: messages in any mailboxes, messages that land in your Inbox, messages from your contacts, or messages from people you've designated as VIPs (see the next item).
In addition, you can also use notifications in Mail rules, as Send Notification is a new option for rule actions. So, for example, you could configure a rule to display a notification whenever you receive a message sent to your work email address, but not your personal account.
We all have particular people--our family members, our boss, our bookie--whose messages are more important than others. While Mail in Lion let you use combinations of rules, labels, and mailboxes to make a particular person's messages easier to identify or view, Mail in Mountain Lion adds a new feature that's custom-made for such purposes: VIPs
(Image Caption: Mail's new VIP section)
A VIP is any person you designate as being important enough to have their messages treated differently by Mail. You designate someone as a VIP by clicking the star icon to the left of the person's name in any incoming or sent message. You'll immediately notice that every message to or from that person now displays a star in message lists, making it easier to find those messages. In addition, when you designate a sender as a VIP, that person gets his or her own entry in a new VIPs section of Mail's Mailboxes sidebar. Click a VIP's name, and you get a list of all messages, across all mailboxes (including Sent and Trash), to or from that person.