Google is testing a very different way of showing promotional emails. In a new experimental view, marketing emails don't look like emails at all; instead, they're big, bold ads with large images. The new view follows Google's controversial move to collect marketing emails and place them in a separate tab in Gmail, away from your personal messages and social network updates. With the experimental view enabled, these emails now show as an endless stream of images rather than a set of ten subject lines.
As part of the new view, Google is offering marketers a way to signal in their messages which image and content to highlight. If senders do nothing, Google will take its best guess at what to show in the grid view. It's notable that this is just a test for now — you'll have to request access to join the "Field Trial" at this link. And if you do get selected, there will be a button in the promotions tab that lets you switch between the two views at any time.
It may seem like an unusual move, but considering that marketing emails originated as a way of getting ads into your inbox, maybe it makes sense that they take the form of a true advertisement.