Post cast time GCD will have to go. It will feel clunky. 2 or 3s cast times are already boring enough to wait on, waiting for GCD after that just won't work. Although I guess perhaps the point is that casting 'big' spells mentally fatiguing so you cannot just keep casting over and over....if instant casts do not have GCD then one assumes after casting a bigger spell you are not going to wait on GCD but instead use instant cast spells before being ready for a big cast again?
As for interrupts, not really seeing a big deal. The graphics show what kind of dent the spell you cast will make to your mana pool. If you move or are interrupted by a mob, no mana is lost. Certain mobs will have special abilities that will be able to interrupt and consume all or a portion of the cast cost. Just a different mechanic, kind of like how in other games there were Interrupts, Interrupts with Stuns if successful, Interrupts with Silence etc. In fact, other games have used a mechanic similar to this - an Interrupt that also drains X mana. Only difference here is that the mana lost will be based on the spell rather than the mob/player ability