I have realized that actual gameplay is very different from the dialogue and worldbuilding. Namely, almost every single class uses mana to perform their skills. It is because of that and the fact only one person has come forward saying they are mana-impaired that I jumped to the conclusion that anyone could use magic, and only a fraction of the population can't.
The following is the most complete text available ingame about how the world's magic works. It is very different compared to actual gameplay. Make of it what you will.
Magic on Lore
Magic on Lore is contained in ley lines, which run across and through the planet like haphazardly-placed latitude and longitude lines. Where these ley lines intersect, there are nexuses of magic which act as reservoirs. There are two central ley lines: the Verix, running North-South, and the Horix, running East-West. To perform feats of magic, a mage must draw on the magic contained in one of Lore's ley lines or nexuses.
Drawing from Ley Lines
The largest nexuses is the core of the planet; it contains the most pure magic available, and is centered where the Verix overlaps the Horix. There is no single mage strong enough to tap into this nexus, though it is rumored the Blue Mage or the Mad Weaponsmith have not revealed the full extent of their power, and they will neither confirm or deny their ability to utilize the central nexus. Normal nexuses and ley lines can be tapped into and drawn from safely, in controlled amounts.
As yet, no regular mage has been known to draw directly from the Core. Doing so would be like tapping a person's heart to draw blood: the pressure of the flow of blood/magic escaping would be overwhelming.
The magician would be flooded with pure magic, overcome in a deluge of power which would burn through their physical, mental, and magical veins. Their stores of mana - magical will and stamina - would be destroyed. They would be "burnt out" and lose the ability to perform magic... and their lives.
Severing another mage's connection to the ley lines IS possible, but dangerous. The potential for lashback - which would negatively affect the severing mage - is enormous. Most choose not to risk it.
Nexuses as Portals
Nexuses, formed where two or more ley lines cross, do not only act as reservoirs of magic on Lore; they can also be portals to the para-elemental plane of magic. On rare occasions, mages can directly transport themselves to the plane through the manipulation of these portals.
You may have encountered one of these portals in Lore already. Legend has it that if a magus looks inside themself and follows the pull of a focus stone, they will be led towards the nearest nexus. In that respect, it is both focus stone and lodestone.
While ley lines are fine for everyday magical needs, if the spell you are attempting requires massive amounts of magic, it would be wise to situate yourself near a nexus. The availability of extra power could mean the difference between life and death. IF you're strong enough to channel it all!
Focus stones, a Primer
Each mage on Lore has the ability to utilize a focus, usually made of a rare stone. These stones can allow the mage to hook into the ley lines and nexuses of magic which transverse the planet, and can guide them to the lines and reservoirs of magic across the world.
With practice and a stone for which they feel an affinity, a mage may also use the focus stone to enhance, to boost, their power. The crystalline structure of many stones allows it to hold the focused power mages channel through them.
As an example, think of a small tendril of a mage's power being funneled into a gemstone. That small jot of power bounces off the facets inside of the stone, gaining energy as it rebounds off the planes. That extra energy boosts power, increasing the strength of the magical working.
With the proper focus and will, a mage may find their strength greatly enhanced, the power of their spells increased manyfold. Of course, that is for normal mages. With the... unique... magic found in the Blue Mage or the Mad Weaponsmith, who knows? These two rare mages have not allowed a proper study of themselves to be made.
Source: http://forums2.battleon.com/f/tm.asp?m=21099744 , bottom of page, in spoiler tag, highlighted.