Spell casts have been removed from the magic system in this rebuild, which currently/previously requires spells of certain levels be cast from certain levels of spell casts available.
Under the old system, if you have two casts available from level 1 and one cast available from level 2, you couldn't cast three level 1 spells.
Now, magic casting is derived from a pool of energy known as mana. For most players (of any game), this should be very familiar and is by far the most widely used magic system.
Essentially, all spellcasters have a large pool of mana points (let's call it 100 points, for example). When it's full, you have 100 of 100 mana. Each spell has a cost to cast, both in mana points and in time. Most of the low-level spells may cost very little mana (so they may be cast repeatedly), but aren't very spectacular in their effect or take a long time to prepare or cast (prepare time and/or roundtime).
We want all hardcore spellcasters (Mages and Wizards, especially) to be able to cast most of their low-level spells without ever running out of mana with sufficient ability-score training. A fighter relies on his steel never expiring, so a wizard's spells should never run dry -- though it's possible to exhaust mana reserves by using more powerful (and more expensive) spells.