I absolutely do NOT want to use the RAW Critical mechanic - a flat 5% either way is idiotic. I just don't know what else to use, though; Brett's suggestion mathematically reduces the odds of critical successes the more dice you devote to something, which is counterintuitive at best. We're already rolling so many dice, I don't really want to roll any more.
Could we just add a "hero point" system that allows us to choose to spend an extra d8 (or so) on a roll. If you succeed on the roll, and one of your normal dice matches the result of the d8, it's a critical success. If you fail the roll, and one of your normal dice matches the result of the d8, it's a critical failure (ALTERNATE: if the d8 comes up a 1 and you fail the roll, it's a critical failure). This puts the risk in the player's hand; you don't HAVE to expose yourself to the risk of a CritFail be simply not spending the "Hero Point"...but you have the potential to get the critical success with the roll by assuming that risk. Say, 1 Hero Point per every 5 full points of Physis, refreshing by 1 point at the top of each session, totally refreshing between adventures, and limited refreshing when the GM feels like awarding it for doing cool stuff?