On AppleTV you can double tap the touchpad on the remote and it will zoom video to full screen getting rid of the black bars on top and bottom of the screen. This does not work in all apps on Apple TV but it is a nice feature and allows me to utilize the full screen of my TV. I realize when I zoom that it I'm losing some of the picture on the sides of the original content but it doesn't really bother me.
It would be nice to have feature in Roku when you hit the * on the remote to bring up the options that it would also have an option to zoom video to full screen.