Cost theory...
I believe Oldtree's method for this was something along the lines of offensive potential, survivability, and ability potential...
It couldn't handle one-off spells, and thus became a problem...
I unfortunatly learn from experience, and this isn't my forte, nor do I think I understand what would go into managing the idea factory...
But I think I could make, if nothing else, a 'Spell theory' that works alongside OldTree's 'Cost theory' while being seperate.
EDIT: I've come up with a theory of sorts, originally a 'Spell theory' but I think one that can envelop all types of cards.
Attack, Survival are creature specific traits.
Ability potential is maintained throughout spells, permanents, and creatures.
Now, this enables Spell to Creature comparisons, which work as thus:
Freeze is 1
to freeze target creature for three turns.
Whereas Artic Squid, costs 2
to use freeze, which makes Freeze the better option, but Artic Squid has the benefit of being repeat-castable (eternal?). This (eternal) effect comes at the price of being tied to a creature with relatively weak stats=1|2, so it can be destroyed by the defensive measure of CC.
This should enable balancing not only amongst creatures, or spells, but also creatures, spells and permanents.
Now, OldTrees, your counter?