On 2009-04-20, Eero Tuovinen wrote:
The Shadow of Yesterday has this same thing you have in Storming the Wizard's Tower, except blown up 100-fold. Players have to be making up rules-elements all the time, starting with abilities and ending with spells. What makes it work is pretty simple: the players making the choices have no ulterior motives, and the system language is clear, clean enough to make the choices simple.
What differentiates between StWT and old D&D is that in D&D the system language is much less structured, and it includes plenty of colorful fluff that might or might not have significant impact on actual play. It's pretty difficult to make good and balanced choices in that environment, while the spell-creation rule you describe is outright simple to use in comparison. As you say, the miscast provides a point of comparison, and creating new effect tracks is easy when you remember that each step should have meaningful in-play significance; I can't imagine going too far wrong with creating spells in that system as long as I ensured that the spells only have variable effects that I could frame into important strategic resource pivot points.