It occurs to me on rereading bkdubs's posts...
Magic needs flavour. Taking a look at generic systems like M&M, there is no flavour. The GM builds a world with none of D&D's assumptions, and as a result it can end up very clear how the setting's supernatural abilities work.
Finding openings in the process of casting a spell should not be any less believable than finding openings in the process of swinging a sword. It's harder to imagine simply because there is so little to base it on; any two people could picture the process of casting a spell differently. We need some set of fluff to base things on, details of the steps involved in a casting. Maybe you can disable one spell because it's cast from the user's eyes, and another by disrupting the flow of mana through the body. The kind of instructions that the Satanic Panic guys have been scouring the books for for decades.
I'm starting a thread for this in Talespinning. Link
here.