I tried plugging it in directly w/o connecting the USB power supply and it didn't work. My set has a USB port so I tried that for power. I initially got an alert that the Fire Stick wasn't properly powered but its been working like a champ.
So ... a problem with using the set's USB port is that every time the set is turned on I have to wait for the stick to re-boot and reconnect. It's very much a first-world problem but it's a little frustrating to wait 30 seconds to a minute for the Fire Stick to work.