As far as I know, there are key combinations for:
Soft Reset
Hard Reset
Launch Windows Feedback (Windows 10 Mobile only)
Screenshot
Other than power on/power off, volume up/volume down, I'm not sure if there's really anything else out there that actually does anything. There's the odd little "press volume up/volume down at the same time" that used to cause the screen to blink, but it no longer does that in the version of Windows 10 Mobile that I'm running (.122).
There's also a combination that is not well known.
Vol+, Vol+, Vol-, Vol-, tilt left, tilt right, tilt left, tilt right, then on the screen: long press, tap, tap, tap, (pause), tap, long press.
This combination will give you unlimited power ups and extend your battery life by 24 hours.
If you haven't figured it out by now, this is a joke, and that combination is the Konami Code. The long presses and taps are the B A in Morse Code. Bwahaha!