As a recent CSL purchaser, the main differences are, the carbon fibre induction kit, carbon roof, door panels, centre console, diffusers front and rear, composite boot, lightweight bucket seats front and rear, thinner glass, reworked chassis, wider track and the exhaust has thinner walls which makes it sound much nicer when the loud pedal is depressed.
Overall a much more focused point to point vehicle than the standard M3, maybe a little too extreme if you are using it as a day to day vehicle but lots of fun !!!