Here's a summary of what we have so far including some things I made up just now. Anything that is not nailed down in the thread itself is subject to change if we think it would be fun or if it turns out Aestrix and I were not on the same page about it.
Worldly Matters:
Momentum is a non-Earth planet with one sapient species, humans. It's a very high-magic environment and, somewhat as a consequence, low-science (the nature of their magic makes it very, very hard to attempt controlled scientific experiments of any scope). It has rings and four moons and so far we haven't decided to introduce any species that don't exist on Earth.
Magic:
Any person in Momentum, even if they're not originally from it, can use magic; the more magic you do, the better you are at it, although this buildup wears off much, much faster than it accumulates and it takes a lot of commitment to be able to reliably do even medium-sized magic. Magic is best at smallish physical effects, it's really bad at information-gathering or mental effects (jury is out on whether this has to do with people in general wanting-privacy and wanting-mental-inviolability at it) although it can sometimes make animals like you, and it can even in basic forms consider things like "repair this thing to an ideal state" to be primitive actions rather than requiring low-level detail work like "attach this fiber to this other fiber". Magic is essentially a mental action, somewhat hard to describe in English but involving clear, focused wanting with something along the lines of entitlement or expectation; it also may react to things like dramatic timing, gestures, or words, but mostly for the effect these have on the caster's mental state.
Doing magic does not directly feel like anything, and people can in fact do a fair amount of subtle magic without ever noticing. However, people who do a lot of it may cultivate aesthetic side effects that they consider appropriate signals that they in particular are performing a spell, so sometimes it looks (sounds smells whatever) like something. Several people who aren't very good at magic all trying something similar-looking on the same target (but not similar enough to all be helping each other) might be legitimately confused about which of them succeeded. In theory people who were better at magic could also be confused in this way, but they tend to have enough experience detecting minute accordance in their targets with their highly detailed wishes and will notice if it's off in any particular, which would signify interference from someone with a different vision. Because there is so little feedback associated with performing a spell, it is not always easy to verify that it went off entirely as planned (as opposed to missing key components but looking the same, or whatever).
Most people never accumulate any momentum to speak of, and while they will find it noticeably quicker to do five small spells in a row than to do the same spells one per day over five days, they will never devote the relentless time investment required to do anything big. Cooperation of intent is possible; a lot of people who want and expect a thing in fairly ordinary quantities can potentially overpower someone with a lot more momentum who wants the opposite. It would for example be extremely hard to pull down a moon, requiring centuries if not more unbroken momentum, because everyone expects and prefers that the moon stay where it is (and anyone who doesn't prefer that the moon stay where it is is unlikely to agree with a would-be moon-remover about exactly how it should go - which means collective intent is better at keeping things the same than at changing them in most cases).
Specialties:
Some people, usually for a career, attempt the same general class of magic over and over again. This is like the momentum-accumulation equivalent of getting a bike - you can go a lot faster, but you're still fundamentally using your own power to cover the distance and it doesn't do you any good if you want to go someplace no one has installed a bicycle path. Someone who volunteers at a hospital five days a week attempting magic healings will eventually be able to do bigger and better healings than someone who does not do this. (This leads to a specialist apprentice-like system for many occupations, where someone who wants to be a healer or an architect or whatever follows around an existing specialist, contributing design help and fetching beverages and doing secretarial work and so on, while being exposed to lots of tasks to want at and contribute to collective intent for, until they have enough of their own momentum to do their own jobs. There is enough value to working in teams that they sometimes continue to do that.) If you do enough specialist work, your metaphorical bicycle path becomes thoroughly paved over and even a medium-sized break from the job won't wreck your extra knack for it, although you can't take off for more than a couple of weeks and expect everything to be there when you go back, and it will be harder to get going again after several consecutive days off than it otherwise might. Having a specialty doesn't make you much better at small spells or other specialist work, let alone make you a proper sorcerer. It is possible to have more than one specialty, although if you have three or more it will become nearly as difficult as outright sorcery to maintain them all, and if you have, say, six, you may well acquire general enough momentum to be a sorcerer entirely.
Sorcery:
Someone who is really and generally good at magic is called a sorcerer. The milestone customarily used for whether someone is "a sorcerer" yet/still in the region where Kithabel lives is whether they can fly - this isn't the sort of thing you can get via specialty because it's not something you can practice without falling out of the sky if you're not good enough (you could try jumping off chairs, but if you don't already have a lot of momentum, the time it takes to land from a jump off a chair isn't long enough to spend wanting-at anything), so it's considered a good benchmark. (It is imperfect because someone could fly around enough to keep specialist momentum for that alone while not doing enough other stuff and losing their general momentum, but it works well enough as a basic classification.) Similar benchmarks in other cultures might include for example the ability to breathe water, but not, say, turning invisible (you could practice turning only slightly invisible with no ill effects if you couldn't maintain it, so you could get it without having generalized momentum).
Becoming a sorcerer is hard. Many would-be sorcerers give up, and others find it necessary to start over many times because they become distracted or sick or they accidentally do too-similar work a few days in a row. Even the amount of time spent sleeping overnight makes a serious dent in generalized momentum; it's possible to be a sorcerer and still sleep eight hours a night (but if you need more sleep, or take a while to get going in the morning, you might be in trouble) so some people attempt bi- or polyphasic sleep schedules until they have enough decades of momentum that they can arrange to need less sleep total. Taking a day off doing magic is out of the question. No matter how much general momentum you have, a couple days of doing zilch will absolutely tank it and one full day off may well drop you below sorcerer level. It doesn't keep nearly as well as specialist momentum.
Approximately sorcery-level momentum is required to permanently affix a magical trait to an object (you may be able to do this before you are able to fly, but not by that much). These objects will only continue to be magic as long as their enchanting sorcerer maintains momentum, and it is customary not to rely on magic objects for anything really essential where a thing that is not magic (anymore) will do. It is possible to enchant people and animals too, but harder.
Sorcerers of sufficient momentum can do basically whatever they want unless another sorcerer or a lot of regular people fight them on it or it involves downtime, but common attributes are sleeping less or not at all, ceasing to age, not getting sick, living in a flying castle conjured out of nothing, stalking around like they own the place, and being highly interested in anything people might want done with magic. (It's not impossible to be a hermit sorcerer and fuck with the landscape a lot, but it's harder to get variety that way.) Sorcerers have considerable incentive to be generously-inclined civil-servant types: if a lot of people want them to stop doing things, they will have some trouble getting anything done, especially if they annoy another sorcerer. It is common for there to be dispatchers who acquire enchanted communication items and notify sorcerers when there is stuff to do.