How much preparation? Priests in 2e needed none. Warlocks needed none. First edition material components add another degree to the mix, meaning you have to decide what to cast and be literally prepared. Rituals requiring components are the opposite. So, it seems there are two divisions here: Spell selection and then literal preparation.
What spells are possible? So, even though a priest may call spells as needed, what those spells are does not change. The opposite would be Ars Magic, or how I was envisioning runic magic; magic that must be creatively constructed at time of casting. This is a little different than selection, because a system might allow for creation of spells but require selection of castable one each day.
How long to cast? This seems minor to me because abstracted time can be fast forwarded and it seems to only come into play as instant casting/long enough to be disrupted. (What if spells took multiple sessions to cast?) I'm thinking now that this an preparation could be combined into how difficult magic is to cast.
What is the cost of casting? If you take away the simple device of Vancian spell slots, how is magic limited? The second most common option seems to be spell points. Psionics takes this approach. Any system involving blood sacrifice is dealing with this feature. Material components hit this too, when an expensive gem needs to be used up in casting.
How learnable is magic? This can be like D&D's % chance to know a spell or Psionics chance of knowing devotions. But it can also be who can learn, because Psionics traditionally could be had by any class. So, this may be two categories, too. Who can learn, and how difficult to learn spells (even if limited to magic-uers). Let's call the first accessibility and the second availability.
How powerful is magic? Old school D&D's system sections spells off into level by how powerful they are. Many spells are affected by caster level too. With old school psionics you could randomly end up with a minor devotion or a major science and yet, the effectiveness of most powers rose with character level. A spell point system might allow variable powered casting of the same spell.
How reliable is magic? Second edition psionics treated powers like proficiencies, i.e. skills. You could fail. This is like Fourth editions chance of stinking cloud to "miss." Older editions have a little of this show up in saving throws.
What does magic effect? Okay, this is probably a lot of categories in one, but I'm thinking of psionics' direct attack powers here. Some magic-users spells are similar, but most affect targets or not depending on saving throws. Psionics has something similar to melee combat, only it isn't. I'm thinking wizard spell duels might fit here. Does magic affect hit points, target state, target stats, etc.
What is necessary for magic? I don't really consider clerics asking gods for spells "magic." But traditional ideas of magic often involved asking for effects from otherworldly powers. This shows up in Fantasy Wargaming and even 2e's Al-Quadim caster type. I'm not sure how to categorize this. Does a spell system identical to 1e's, but requiring demons look any different in practice? Maybe the difference is the implied risk. So, this would be another possible cost. Maybe we could call it caster risk.
As always with categories, we could probably combine some of these and divide others, but to reorganize these into a draft list:
What is magic?
I'm at work with no dice, but let me try a web dice roller:
That looks like: Although possible to create new spells, spells are pretty much set. Spells probably have no saves or rolls to hit, might even function identical to melee combat. Only magic-users can use spells but they can use pretty much any of the set spells. Preparation is a pain, probably a system of rituals. Few limitations, which makes sense if there is a lot of cost already built into the preparations. No risk at all. Spells are super reliable and very powerful.