I DM probably 2/3 of the time I guess, but me and the other person who DMs are almost the exactly the same.
No reasonable request will be unreasonably denied.
That sums up what we allow. Unless there is a clear in game conflict that will arise in game (like the player wanting to play a half dragon with lasers), then we're pretty open to at least hearing the player's ideas. Then we work it out as best as we can before the game starts.