This really ought to be a linked topic reply to this post, but I don’t seem to be able to do that right now.
It looks like tilt is broken on Windows. MyPaint’s use of tilt is wholly dependent on GDK reporting tilt to it correctly (or at all).
GDK is a lib we don’t directly control. However it’s open source and you can send patches to its developers. It is possible that the init-time test in gdk/win32/gdkdevicemanager-win32.c is being too strict about letting tilt through.
It’s trying to interface with an ugly proprietary thing called WINTAB.dll, which is basically a bit of the tablet driver that presents a supposedly fairly uniform programming API for tablets, albeit an old 16-bit era one. Most manufacturers have chosen to support it, which is good until you realize that things like MyPaint will only really support one of your tablets at a time unless your drivers support a range of tablets (Wacom’s seem to, other manufacturers I’m not so sure about).
There’s a worrying comment in the GDK code’s decode_tilt(). It would be good if someone with time could look into this area too:
/* As I don't have a tilt-sensing tablet,
* I cannot test this code.
*/
We have a ton of information about making debug builds of GTK/GDK on our wiki right now: Debugging […] on Windows. If you’re trying to get tilting tablets working again, you may want to set DEBUG_WINTAB in the compiler when you build 
