Mail's layout is optimized for the wide screens that all Macs have these days. By having both the message list and the message view in a column layout, you can see more of each of them. There is almost no need to actually "open" a message.
Mal is also one of the few apps that I think runs really well in full page mode. Almost every action you need to work with your email is either on the tool bar or the context menus. I rarely find it necessary to access the Menu Bar.
At first it bugged me that the list of mailboxes is hidden by default, but I soon realized that I rarely need anything on this list and it is nice to have it out of the way. If you need to see the list of mailboxes, simply click the "Show" button on the top left.
When I do want to access a mailbox other than the Inbox, it is usually just one of a couple including, Sent, Drafts and a couple of smart folders. Lion's Mail has added a Shortcut Bar across the top of the window and you can drag any mailboxes from the mailbox list to the Shortcut Bar. (This is very similar to the Bookmarks Bar in Safari.)
In my opinion, Mail is the very best of the Mac's philosophy of getting the interface out of the way so you can focus on your work.
If you find that you really can't warm up to Mail's new layout, you can change back to "Classic layout" in Mail's preferences. Before you do that, however, I recommend that you give the new layout a fair trial.