What is MTP then? it’s simply what’s been used in One S, One X and One X+ that’s running 4.1.1 (jelly bean). Stands for Media Transfer Protocol and is developed by Microsoft, and as you would guess it works very well on more recent versions of Windows.
But, you need an additional download on Windows XP to get it to work. Forget it on xp. And on mac? Sure, you can get it to work if you download the file needed for it.
Edit: MTP is also used in the new HTC One.