Law of Conservation of Reality
During the first Mage Wars (shortly after the creation of humanity) magic was not as it is in modern times. It was unbound and not subject to any sort of rules. This meant that it was quite an effective weapon when wizards, blaming the Gods of the Disc for their situation, declared war. The resulting magical chaos threatened to destroy the Disc (and reality alongside it), had the Olden Ones not intervened to stop it. While they forced the Gods to acknowledge a higher authority, they also saw to it that the magical might available to humans was diminished. They bound magic to obey the Law of Conservation of Reality.
Quite simply, this law says that the effort required for an action using a spell must be equal to the force required to perform the action using more conventional means. For instance, in The Light Fantastic, when Archchancellor Galder Weatherwax wishes to levitate to the top of the Tower of Art, he must remove a rock from the tower's crumbling top in order to do it. Another example, given by Rincewind when trying to explain this to Twoflower, is that making an illusion of a glass of wine (or anything else) is easy- it's just manipulating light- but actually creating a glass of wine takes much more power. As mentioned in both cases, if a wizard tries to break this law, his brain would be pushed out his ears due to the forces involved.
Witches are presumed to be bound by this law as well (even though their magic is different from what modern wizards use) but a handful of other creatures are not. The dryads are one of the few races that still have access to what they call the Wild Magic and use it impressively - for instance, Druellae and a circle of dryads teleport Rincewind outside of their tree into the Temple of Bel-Shamharoth in The Colour of Magic. Another exception are the Sourcerors, who actually create magic and therefore have all the power they need to do whatever they want. Finally, both octiron and octogen have some of the ancient, raw magic in them, which is why they both radiate a strong magical field.