duntmatter - a swingarm is unsprung mass, fork lowers are unsprung mass - it's the bit that moves.
Wheels are unsprung mass.
if the moving parts / unsprung parts are lighter, they will move quicker, so the suspension will be more responsive.
MrGnar, The earth has loads of rotating mass, it's not slowing down very much. i'm happier riding Dh stuff on a bike with heavier wheels because the bike tends to hold a line better, it feels less nervous.
so for me, there's a dilema: lighter wheels = better suspension action, heavier wheels = more confidence through tricky bits.