The main new feature in the Cube is that it includes far-field voice recognition. In previous iterations of the Fire TV and the Fire TV Stick, Alexa was included in the remote control, where you'd have to hold down a button to access the voice assistant. Now, you'll be able to call it from across the room without pressing a button.
What's also worth noting is that the device has a port for a wired internet connection. Ever since the latest model of the Fire TV was introduced in September, Amazon hasn't offered one with an Ethernet port. This is good news, as you might want the additional stability of a wired connection for 4K UHD streaming, which the Fire TV Cube does support.
You can purchase Amazon's new flagship Fire TV set-top box now, for the price of $119.99, and the company is currently offering free one-day shipping for Prime members.