Did you let it go for a while? Once everything is downloaded, it uncompresses and installs. It can look like it has stopped. Try that and meanwhile I will check to see if we have any reports of failures to install.
>update< Ok, I checked with the specialists and they say there are 3 problems that can cause this - an issue with the connection to the XB store, a firmware problem or a faulty internal drive. Let's start with the easiest one, which also is more common.
The recommended steps are:
1. nuke the existing partial install
2. Do a hard, power cycle, reset of the console
3. Verify your network connection and reset your router if the connection is not good/solid
4. Reinstall the game
If it was the firmware problem, it could require reinstalling the OS, so I hope it is not that. If the drive is failing, you could start seeing other odd errors.