Originally Posted by RyanB
??? That may be the case, but what then prevents a user from deleting system files? Is it only the /data folder that's visible?
From a file storage/app point of view, the internal storage will only be visible in windows explorer. The user doesn't have access to system files. You will be able to see the entire contends of an external SD card (as long as you have "show hidden files" enabled).
A little history ... when Android first hit the mobile market, most phones had 512 MB of internal storage and expansion was handled only through an external Micro SD card. Then the card would mount at /sdcard and app developers would use that for storage of large data files so not to exhaust the limited internal memory. As phone memory began to expand beyond the 2 GB mark, manufacturers would create an internal partition called /sdcard to take advantage of faster, more stable internal ram. If the phone also had an external SD card slot, then the manufacturer would mount it at an arbitrary point like /sdcard/external_sd or /sdcard_external. Even with more ram internally it was still partitioned between the system and storage and too many apps could cause problems with space in the system partition. Because apps were looking for the /sdcard mount point the external sd card became pretty much file and media storage only.
With Jelly Bean, /sdcard is virtualized which means that the entire internal memory can be used for both apps and file storage, relieving a lot of the limitations of fixed partitions. The downside is that a factory reset now deletes stored files as well as apps and user settings. Nandroid backups are a little funky, too, but that's a bit more complicated.