Kassadin's Magic Shield
Should block any CC that comes with the magic damage.
Makes no sense to trade Q with
if I'm still going to be stunned or binded afterwards.
Post Note After Reading Comments
The shield should only block magic damage CC, not attack damage.
Meaning, a well timed Q from
should block the stun from 
(fear) but not block the stuns from

It should block and mitigate CC from
blind, but take full damage and still be blinded by
because Quinn's blind is physical damage.
CC that doesn't have damage tied to it, like
pull will also work. Though Nocturne's fear can be "blocked" he can still be feared by
because there isn't magic damage tied to it.
To simplify, if it does magic damage, you should be able to block it completely, damage and CC. everything else is ok for him to be vulnerable to.