A door of two choices... Everypony must choose one.
Light or dark. Each single action can influence another to cause a chain reaction.
Sometimes, both choices are the wrong one.
Sometimes, both choices need to be chosen.
[Created for my friend, sunshinemonster]
Were it so easy.
I can see Starswirl choosing the dark door. After all, that's where the greatest number of mysteries would lie concealed. Plus, most ponies would choose light; there would remain few surprises there.
That, or he'd go looking for what lay between the doors.