We all know how annoying it is to have to take time and cast a buff spell, only to have it disappear on us or go awry when it's of the most importance. So instead of simply casting a spell that runs out on a duration, it would instead act on a toggle. And instead of simply spending your current magicka on it, you would instead have your total magicka lowered. This lowered amount is the power required to sustain the effect over time. Too many sustained effects, and you have a very small pool of magicka to work with at any given moment.
Most sustained spells would probably be player buffs, but others could include party buffs (any allies within the character's "aura"), enemy debuffs (also in an area around the caster), or even summoned creatures.
Thoughts?