First of all, themes in WP.com are built differently from the ones for self-hosted users, and this demo http://wpshower.com/demo/?theme=suburbia in WPShower is the self-hosted version so there are several differences from the one you are using in WP.com here.
Having said that, you are correct about the changes about the theme in WP.com. When Suburbia launched in here used to display a featured image in a single view but it's been updated shortly after the launch to not to display. The reason behind this is that, in this way, users will have more control what image goes the top of the post. Sometime users wish to have different image at the top of the post.
You are always able to insert the image into the editor to show it at the top of post.
I hope it clarifies the update.