The second biggest complaint after Outlook 2013's lack of color, is the size of the header in the reading pane. Yes, it is huge, approx. 70 pixels taller than in Outlook 2010, which was taller than the header in Outlook 2007. To make matters worse, the header can't be minimized.
While it's not bad when the reading pane is on the right (where approx. 75% of users keep it), when the reading pane is on the bottom, users are lucky to see a line or two of a message with the pane's height about half the height of the window. Disabling Contact photos won't help and in fact, adds about 6 pixels to the height. Add attachments or the blocked external content warning, an Outlook App and the People Pane and you're lucky to see any part of the actual message when the reading pane is on the bottom.
Part of the reason the header is so large is because of the reply and forwards buttons, photos are a little smaller than in Outlook 2007 and only account for half the increased height. The rest is taken up by more white space.
The App bar shown in the screenshot is available only with Exchange 2013 mailboxes. The apps are available in the Office App Store.
The obvious solution is to use the reading pane on the right. The reading pane works great on the right and there is plenty of room to read messages, even with the ribbon open. With the long list of messages visible onscreen, it's productive. Where it doesn't help is with portrait orientation - I like the reading pane on the bottom so I can keep the Folder list visible but the header takes up a huge chunk of screen real estate.
This could be time for third party reading panes to make a comeback. Microsoft is aware of the problem but whether they will properly fix it is left to be seen.