At the very bottom of the message viewer window there is a pair of lines with a dot between them. Move your cursor over the dot and the cursor will turn into a short horizontal line with an arrow pointing up. Drag upward and once you've gone far enough (in my window it's a couple of inches) that dot and the pair of lines will jump up to create a message viewing area below, which will display whatever message is currently selected (or "No Message Selected" if none is).
(At first, I didn't think this worked anymore. My preference is the opposite of yours—I like the main Mail window to show only a list of emails and no preview. When I tried dragging up from the bottom nothing seemed to happen, but I found I just wasn't going far enough up.)
