Android (since Honeycomb) switched from USB Mass Storage to MTP/PTP.
I understand the reasons for (some) switch, it's just simpler to avoid filesystem-mess when your average windows starts crapping at that exFat. In general, it makes it easier for device developers to figure out partitioning, since they don't have to unmount the drive before making it accessible to the computer.
Now, there's two points I absolutely don't understand.
First: Why the HELL would you completely take out any support for Mass Storage? Every PC has it. Most smartphones have an sd-card that could be unmounted without any concerns. At least let me mount my SD card!
Second: WHY MTP? Has anyone noticed that the current implementation of MTP clients is absolutely horrible on the PC side? I don't know about Mac, but MTP on Windows has some horrible memory leaks (yup, i really got a complete system crash from just copying files), it's terribly slow and most important: it's not implemented as a mount protocol, but as something else you can only access through explorer, not through your average sync program.
On Linux it seems there are better implementations with FUSE, i might just have to make the switch - oh wait - there's no good music manager for linux, so why bother (let's not discuss that).
So while I am stuck with taking out my SD card whenever I have new Music, maybe someone will come up with a way to make my sgs3 enable UMS again. I mean seriously, this is a huge step backwards.
/edit - Quick Update:
I figured out how to work around Samsung and re-enable USB Mass Storage. Wee.