Player (wants to stay anonymous): ...and and my fighter Kronkart The Mighty has to be vegan!
DM: But...?! What? ... eh ... Why would he be vegan?
Right answer: Because then he gets SUPERPOWERS!!
Point in case*
Here my imaginary DM would applaud and say somthing like: Bonus points for using nerdy movie knowledge while creating a character. I wouldn't allow superpowers, though. Well, I could offer you pointy ears and infravision...
Wrong answer (in my head, anyway): I'm vegan in real live and I wouldn't like the idea of having my character eat meat, you know?
And the DM in my brain would say: I wouldn't want to hurt any sensibilities, but it's only a game, goddarnit! It's for playing and having fun, you know.
Then I would smile and think about some rules instead.
*If you haven't seen Scott Pilgrim vs. The World yet, I can only recommend it. This movie is AWESOME!!