Share 'V-Brake Enlightenment Question'

I contstantly hear about "clearance issues" when going from cantis to v-brakes.  I'm not really sure what that is referring too.  Is this clearance between the pad and rim?  Is is clearance when they're opened up for changing wheels?  Is it a worry that the brake arms/cable/noodle provide a smaller opening and therefore may catch more mud?   As I see it, the distance between the pad and the rim w…