I tend to define my graphical style first, since I find that it trends to direct the mechanics of the engine.
If I have cartoony graphics then the engine can be more loose, and exaggerated in it's allowances... where as something more realistic looking may dictate a tighter feel or a more brutal approach to outlining core mechanics.
I know this is completely the opposite to how most people do it... but that's why I'm me and not them.
~Sol