One question is, what happens to excess mana if you're doing something that only requires so much before adding more becomes meaningless? For example, if you're teleporting something, the person is either there or isn't (although it'd be amusing for teleportation spells that -don't- have enough mana to go horribly, horribly wrong). Does the rest of it just disappear? Seems like a conservation of energy problem.
I really like the idea of raw sphere-aspected mana affecting things in dangerous ways. I don't think you'd be able to have different levels of aspected mana in flows, as there are a lot of spheres and eventually Toady will probably allow you to add your own. But I think that for situations where there's excess mana for any reason (either leeching off an unskilled caster's spell, or from the above situation), magic could manifest itself as simplified creatures with sphere-based behavior. Think ball-lightning, appearing without warning, flying around mysteriously, and disappearing just as quickly in a flash.
When one of these "creatures" (let's call them manifestations) forms, they wander around for a while until they hit something they can affect. They may be attracted or repulsed by things aspected to specific spheres, specifically I'm thinking opposites would attract them because high levels generally flow to low ones. So a manifestation of Salt mana would move towards things aspected with Water (like blood), but away from Sea or rock salt blocks.
They can move differently or look different, using effects like electricity spreading and reforming with lightning mana, or water mana creating a bit of actual water around it as it moves. They could even have different physical properties - many manifestations may fly, but it'd be more thematic if a manifestation of Minerals would creep along the ground, encrusting things it passes in stone and crystal. This stuff would be possible with a reasonable amount of CPU load because it's not always present. Regions might have a certain amount of ambient aspected mana but unless it's crazy rich in the stuff, you won't see it happening just whenever.
The other thing I think would be cool is if fragments of the spell that created the manifestations could travel along with them. If you're casting a lot of different spells, this could be especially dangerous, as manifestations could attract to each other and create new ones outside your control - imagine a foolish mage over-casting "burn wood" and then "heal friend", and having manifestations with the words "burn" and "friend" combine. Who is the manifestation's friend?
Better, what happens when a manifestation is aspected with a truename? It might last a lot longer, becoming a sort of simple ghost, as truenames are supposed to be somewhat permanent and intrinsically powerful. It could grow into sort of a curse for the person, flying around and absorbing other manifestations, building up mana until a critical point is reached and the spell releases.