This is a big shift back to the original Xbox era, where the internal hard drive was also not user accessible. On the Xbox 360, the company went to a hard drive model where the user could replace or upgrade their hard drive. Granted, you had to purchase the hard drive from Microsoft, but you still could upgrade the hard drive (as I have done myself to the 320GB hard drive I have on my Xbox 360 now).
While Microsoft admits that the 500GB hard drive will be used for system software as well as updates which will take space, the company is offering full support for USB 2/3 external hard drives that users can purchase that can be used to store all types of content for the Xbox One. Other than the external hard drive needs to be one with a power supply, it does not seem that Microsoft is going to put any restrictions on the size or brand of drive that can be used. So, an external top of the line 7200 RPM USB 3.0 hard drive would seem to be the drive of choice for serious players.
With the Blu-ray drive used in the unit and the Xbox One requiring hard drive installs, it would seem that large hard drive external storage will be a must for the serious Xbox One gamer.