If you really miss the old iChat interface, don't worry: You can hit Command-1 (or select Window -> Buddies) and you'll see a buddy list like iChat's. From there, if you double-click on a contact, you'll be taken back to that right-hand conversation pane, ready to chat with that person.
As in iChat, you can do group chats in Messages, but the process is slightly different: In iChat, you could select multiple people in your contacts list for a chat. You can do it that way in Messages, too, using that Buddies list. Or you can do it in the primary chat window, by entering multiple names, numbers, and email addresses in the To field. When one person replies, everyone else sees that response.
If, on the other hand, you're having multiple one-one-one conversations at the same time, you can open individual windows for each one by double-clicking on each contact in the left-hand contacts list. That may be easier to manage than juggling a bunch of conversations in a single pane.
If you click on the little triangle next to a contact's name in that To field, you get a list of all the ways you can reach him or her--all the email addresses and phone numbers he or she has registered with iMessage as well as AIM and other conduits.
Apple has cleaned up some of the menus in Messages. So, for example, instead of New Chat and New SMS in iChat's File menu, you have just New Message--blurring the lines between the different messaging platforms the app supports. You might notice the same thing on the Buddies menu: There's just Start New Chat instead of Invite to Chat, Send Instant Message, and Send SMS.
One other interface difference worth noting: If you go into the Accounts preferences pane, you'll see your various accounts. Most of those account options are the same as before--except for your iMessage account (assuming you've set one up). Standard AIM accounts have a Privacy tab, where you can specify who can or can't see that you're online and send you messages. (The options include Anyone, Anyone in My Buddy list, or specific people in your Contacts database.) As a result, when you're using iMessage, you're perpetually available to anyone unless you specifically block or ignore him or her.
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Perpetual availability isn't the only change iMessage brings to the Mac. As in the iOS app, you can opt to send people you're chatting with read receipts. (That does make it a bit harder to duck out on an awkward chat.) It also means that you can see when the other party is composing a reply (and they can see the same for you). All of those iMessage conversations are encrypted from end-to-end. You can, of course, use the iMessage system to send photos, videos, and other multimedia file types; it allows attachments of up to 100MB.