Actually, a lighter flywheel will make it easier to accelerate. This is the same concept as lightweight wheels. The reduction of rotating mass causes a lower rotational moment of inertia. Essentially it requires less force to accelerate something that weighs less.
The disadvantage of a lightweight clutch or wheel for that matter is that it is also that it is easier to accelerate.
The same property that makes it easier to accelerate to a higher speed makes it harder to remain at the same high speed. The flywheel has less momentum and since the engine and tires and whatnot are still producing the same amount of friction, the flywheel cannot use its rotating mass to keep the engine turning as easily.
The disadvantage of having a lightweight flywheel can most easily be seen when you are cruising along and come to a hill. With a very heavy flywheel, the rotational momentum helps keep the engine rotating at a constant speed (it wants to remain at a constant velocity), whereas with a light flywheel, the added force opposing foreward travel can slow the car down easier because there is not as much rotational momentum keeping the engine turning.
So yeah, that's that. This is all basic physics guys, don't let it fool you.
