nope, nothing bad will happen to your computer. Neither will it affect your game
i often use it when i need to get out of something quickly, but if i can bring up the task manager i have found that much quicker and i prefer that when i can use it.

It's a fun hint, and for those unknowing of what it does, atleast it gives them a new experience to keep in mind next time, just in case another tells them to do it. Though Alt+F4 is common knowledge these days, i guess.

i dont think it works for full screen, i havent tried though O.o