Mass storage doesn't work on a device like the Galaxy Nexus that emulates the "SD card" on its internal storage, because it requires making the blocks of the underlying device available to the host OS.

IMO it's worth it to avoid that awful old chestnut where you'd run out of internal space with heaps left on the card, but unfortunately MTP isn't quite as convenient :(

The issue is that new devices don't have filesystems that can be exported over USB mass storage. Instead of having a small data partition and big FAT32 external storage partition, they have a single bigger data partition with a media storage directory.

The data partition is ext4, so can't be mounted by most operating systems. It is also shared between apps, app data, and media storage so it can't be exported while in use.