How do I enable an auto download of newly installed Metro Apps?

There is currently NO functionality to automatically "download" apps that you have already purchased on other devices. However, your app states will sync across devices. For example, the Kindle app knows which page of Shades of Grey you were reading on Device A. Open Shades of Grey on Device B, and voila! Pick up where you left off. To my understanding, MS deliberately opted to NOT automatically download apps based on account because there will likely be many apps that you will use on your phone or tablet but not on your desktop PC. Wordament is a good example for me. I play this on the phone and tablet, but don't have touch capability on my desktop, so the game is nearly impossible to play. Here is a great article on Sinofsky's blog if you're interested. Signing in to Windows 8 with a Windows Live ID

windows-8 windows efficiency app

Looking at these pages from Microsoft and the Windows App Store, it doesn't look like there is any sync option right now.

Both this page and this page say that you can install apps that you own on up to 5 PCs, but they also specify that you must do it from each computer you want to install the app on. "From the list of the apps you own that aren't installed on this PC, choose the ones you want to install, and then tap or click Install."

So unfortunately, it doesn't look like this functionality exists right now.