How well do you support keyboards (multiple, international or strange ones in particular) and USB game controllers (particularly enumerating, mapping and calibrating unpredictable axes, switches, buttons)? I'd like something more friendly than the low level Windows API but more general than Microsoft's arrogant assumption that every peripheral is an Xbox 360 - like gamepad.
EDIT: I looked at the API docs. My grades: E for documentation quality (all skeletal, Murl::RawKeyCode deliberately omitted), F for keyboard support (half bizarre and half undocumented) and D for joystick support (Microsoft style).
Given the halfhearted documentation and the missing Linux support I have the impression that you have gone public too soon with an unfinished and unproven engine; a showcase of finished games would give a better impression than a roadmap of important features that aren't available yet.