I don't think this a sign that Apple's moving away from drawers. Rather, I think it's a sign that Apple is thinking more about how they should and shouldn't be used. It never made really good sense to use a drawer for your mailboxes to begin with. Drawers exist to house items that the user is likely NOT to want available, so they can easily hide and show them. Does anyone hide their mailboxes? I seriously doubt it.
Also, the change from right to left makes sense, because it keeps it in tune with all of Apple's other products; iTunes, the Finder, Safari (managing bookmarks), and iCal all use the left-to-right style. The container is on the left, the contents of that container on the right. It just makes sense.
That said, I find the Mail's new look absoulutely horrendous for a variety of reasons, and I'm having trouble adjusting the left-aligned mailbox list, too. I'll get used to my mailboxes at the left (and, like I said, I think it's better that way, all things considered), but I'll never get used to its overall look & feel. I think Apple dropped the ball on this one.