I selected "mana points", but that's really not true. What I really mean is a drain-type effect like SR, or some other "cost" associated with magic. Nothing prohibitive, just a trade-off.
A true "mana point" system is really just quantum spell slots. Kinda "roll your own" slots. It's more flexible than true slots, but still has the same lame "uh, I forgot" flaw that slots have.
When I was pondering writing my own spell system, I was looking at one where casting was skill-based, with failure having potentially bad side effects and taking a bit of time. As an option, the wizard could "hang" a few spells. Hanging spells still involved the skill roll, but it was under more controlled, safer circumstances. Once hung, a spell could be released later without a chance of failure. Any hung spells could be detected with a Detect Magic or similar spell. Also, the energy from hung spells tends to interfere with casting/hanging of other spells and raises the DC for any further attempts in a cumulative manner.
One thing I'd love to see in a magic system is to have material components actually matter. The way D&D does it, now, they are barely even window dressing. Might as well ditch them.