Microsoft has recently confirmed that, just like Sony's PlayStation 4, the Xbox One will have the capability to play games after they've downloaded a certain number.
According to Polygon, the Xbox One was already revealed to be able to play games as they're installing. However, it was never stated whether or not this would include Xbox One games that were being downloaded.
Considering that Microsoft's original approach with the Xbox One and its online capabilities were to leave behind discs, steps like this are exactly what the company needs to take in order to wean console users off physical media.