Let's see, my favourite starting motivation is 'let's go on an adventure!' and see where it goes from there. Yes it's simple, but it gives the character some excitement about the situation they are in (although they might grow to regret it later), it's playful, and it doesn't give the start of the experience an air of obligation. It also fits almost any setting.
As for the standard motivations, well, I can definitely see why duty or threat to PC's life are widely used, they leave most room for interpretation of the character, since unless they want whatever horrible fate to befall them, they have no choice but to participate. I don't mind these, since they tend to lead to more fun shenanigans.
