Thinking about old-school magic

So my Old-School Psionics book was pretty well received, and I've been thinking lately about something much more radical: designing a new magic system for D&D.

My basic starting point is to eliminate spell slots, and replace them with something a little less certain- meaning that spells won't be "fire and forget" like SAMs.

I think such a system would be better suited for low-magic games, something I am a long-time proponent of.

Of course, this includes most of my favorite fantasy settings, such as Hyboria and Thieves' World.