However, I want to point out my favorite thing, which is how I've resolved the problem with mana/spell points/tallies. I've stripped down and rebuilt the thing several times, and I'm really happy with the results. Here's a summary (in case you don't feel like perusing several pages)...
You have a pool of spells each turn (1 minute) equal to your level. This is your mana.
Each time you cast a spell, you have to check your linked attribute (for example, persona for a light caster) to see if you spend a point of mana. The more powerful a spell is, the more likely it is to cost you a mana point. The DT of the check is 9 + the level of the spell. On a failed check, you spend a point from your mana pool.
The coolness comes with scaling.... let's look at healing word. This is a simple light spell that restores up to your level d6 hits. However, you get to decide how much risk/reward you are investing with every casting; if you restore 2d6 hits, you need to succeed with a DT 11 persona check or you have spent a point of mana. If you restore 5d6 hits, you now need to succeed with a DT 14 persona check. Since you're probably going to have a linked casting attribute of 6 (primary casters will always maximize that attribute, no doubt), a level 1 spell requires a DT 10 check (meaning you are successful on a 1d12 roll of 4 or better), while a level 6 spell requires a DT 15 check (meaning you are successful on a 1d12 roll of 9 or better). High-level spells are likely to use a mana point, while lower-level spells are less likely to do so. Because of the narrower range of the d12 as compared to the d20, a decision to even roll 3d6 instead of 5d6 on a healing spell increases the likelihood of keeping a mana point by over 16%. This is a genuine
It ends up replicating a spell point system, keeps the best of the previous tally system I had employed, and makes magic a bit more mysterious, since nobody knows for sure when you're going to run out of power.
By level 6 or so, you're probably going to make it through most combats without spending all of your points (although deciding to do nothing but drop level 5 and 6 spells might eat up those points quickly), but a playtest will reveal more to me.