As I mentioned before, this is gona look kinda sketchy, but you are just going to have to trust me that there is a complete system lurking in the background somewhere.
The primary reason for changing the rules for magic, is because magic was changed from the basic AD&D type of game. There are several differences, the two primary ones are frequency and entrophy.
Despite the fact that 5 out of the first 8 player characters in the campaigne were able to work spells of one kind or another, magic is much rarer than in other campaigns. The average peasant has never seen a creature capable of working magic (not that he would know if he did), or any type of magical beast. Most clergy are not priests (there is a distinction) and magic items exist only in dim legends of a few mighty items wroght by gods and wielded by kings. And phrases like: "Oh there goes a mage casting Hold Person in a bar brawl again..." are never heard.
The second difference, entrophy, reffers to the fact that magic is (in some sage's opinions) composed of the raw energy of chaos, always proceeding towards the least ordered state, usually contrary to what the wizard wants it to do. In any case magic is much harder to control, and many spells go wrong, with often, ah interesting, results.
The above two changes make magic harder to use and harder to find, generally bad news for the players who rely on magic for everything. To illustrate the difference, in the campaign to date there has been one request by a player for another player to use magic, it was a life-or-death situation and the requesting player, as well as the caster both cringed for the entire process.
A third difference that might seem merrily cosmetic is the fact that no mortal may ever create magic. Any mortal spellcaster creates the magical effects by maipulating pre-existing magic coming from one of 4 sources:
1) Greater Powers: Gods and their kin, being really immortal, can do whatever magic they want, and will occasionaly give ths power to others. These others, include most priests and a few other types of spellcasters, usually do not run the risk of their magic doing something unexpected, but the powers that be are fickle in their bestowing of power.
2) Lessar Powers: Deamons, Spirits and Souls. These and other creatures who are immortal, but not of the stature of gods can also be forced, coerced, bribed or begged to give magical power to a mortal spellcaster. These creatures can be made to use their own abilities for the wizard, or to teach him how to manipulate symbolic links.
3) Symbolic Links: These include formaulae, symbols, shapes, various materials and words of power, which already contain magic in their essence. Magic can therefore be invoked from the items which already contain it, the process becomes one of simply releasing the magical energy, and controling it in the process. This prodeedure is used by many spellcasters, exclusivly or as a crutch to make other methods easier.
4) Existing Patterns: These include elemental forces, alternate realities and possibilities and magical items. Some magic is found naturaly in patterns surrounding the earth, or in previously enchanted items. All that needs to be done is to shape it to the particular situation.
Spellcasters who are not mortal may create magic withing the bounds of their abilities, creatures such as these include the fay, with their use of glaymor, and dragons.
This may seem increadibly limiting for a wizard, but there are some good points, as well as more hangdicaps, but those who play wizards for the roleplaying and not the power shouldn't mind.
As can be seen the line between wizards and priest has become a bit blurred. The only important condieration is how the spell effects are resolved, which is different for priest and wizards. The druid for example, follows most of the rules for the priest class, but his spells are resolved as a wizard.
When a wizard casts a spell he roles d20 and adds the following to it: his level, any race or class modifiers, and a spell modifier based on the level of the spell. If he gets above 20 the spell goes off in the normal manner. If it is far above 20 it may go off better than normal. A result below 21 means that the wizard cast the spell incorrectly, and the DM roles on a percentage table to determine the actual result. The amount by which the wizard fell below 21 does modify the percentage role. The result of the percentage role may cause the spell to function nearly normally, not at all, or in some truely unexpected maner. Often the unexpected result is bad for the wizard who often experiences a backlash of some kind. On the up side, a wizard does not memorze spells, when he wants to cast one he does so, providing he is not too fatigued to do so. In addition many wizards have other magical abilities besides spellcasting. All wizards are required to specialize in just a few of the schools of magic.
Wizards generally have some method of researching their own spells, which will probaly be the primary method for most player wizards to gain spells. My policy has always been that players should be allowed to research slightly more powerful spells, since they are genrally more limited, as well as being the exact spell that the player wantrs, and not just the closest thing which the PHB had to offer. After all most wixards in mythology spent long years in isolation figuring out a new spell, not going around killing other wizards so they could learn spells from their spell books.
Priests also need not memorize spells, and like wizards simply pick the spell they want to cast at the time of casting. At that point the similarity ends. Priest have a chance of failure based on their wisdom, which is read as 4 points lower, and always has a minimum of 5%. This is the chance that the god or power isn't listening or decided to withhold the spell for whatever reason. Priests also do not know what their progression is, and unless they cast spells very often will never know how many they have left. The thing that prevents priests from casting spells all the time it that their progression is affected by the length of time that has passed since they have last asked their god for a spell. The longer the time, the better the progression (Anyone remember that priest in Ravenloft, Aaron I think his name was, who had NEVER cast a spell, and then finally cast his first spell, Raise Dead?). At any rate this should give a priest plenty to think about. Priests also may have other abilities given to them by their god, which are handeled in other methods.
There is one other final difference in the magic system that I will tell people ahead of time. Many of the spells, primarily wizard spells are different either in their workings or level. Since there are no alignments, spells which detect, hide or modify them are either nearly usless or much changed. The find faerie rule also modifies a number of divination and illusion spells. On of the broadest changes in spells is the fact that the spell level is determined not by the effect of the spell, but by the dificulty in casting it. The difference is that a lot of the spells which create something from nothing, either matter, energy or an effect, are much harder to cast than their effects would indicate. Conversely, spells which deal with existing matter, energy or effects are much easier to cast. As a result manny invocation/evocation spells, such as magic missile are up to 2 levels higher. Since magic missile causes raw damage to appear without any chance for defense at a distance to any target, it is put on the same level with alteration and necromancy spells which cuse more damage, but are harder to use or more limited. The justification for this change, is the fact that in mythology and legends, these types of spells were very rare and used only by the most powerful wizzards. I have also found that from personal experience that the mass damage types of spells tend to discourage roleplaying solutions in favor of "toast them all and let the DM sort out the XP". On the up side there are a couple of spells such as the monster summoning family of conjurations which are a level lower, although they do not work quite the same.