Well, because Erno asked so nicely.
The dreamworld, or limbo, is a universe parallel to our own. Dreaming people are observing and subtly, but constantly manipulating that universe, and as such, it is a bizarre place, where no laws apply. As the dreamworld is not exclusive to human dreams, it contains items and entities no man can understand, in addition to warped and mutated monstrosities, objects or places that can cause insanity simply by looking at them, and so on.
Dreamcasting is the art of projecting the power of the dreamworld into reality. The caster enters a state of very lucid dreaming, in which he perceives both the dreamworld and the real one. The caster then uses his consciousness as a portal to let a select item, entity, or idea enter the real world from the dream world.
While the caster can manipulate such a dream-thing to an extent, he is still limited to what he finds once he enters limbo. In addition, thinking dream-entities might try to use the caster to escape limbo, by force, diplomacy, or other means.
Another disadvantage of dreamcasting is the use of the caster's mind and body as a carrier for dream-things. Human brains and bodies work great in this physical world, but they weren't meant for the insanity of the combined dreams of everything that ever lived.
Every dream-thing that passes trough the caster's consciousness changes the brain, causes it to question what used to be facts, and accept as reality what it saw in limbo. Depending on what knowledge erodes and what weirdness is added, several sorts of insanity are possible, ranging from almost all known phobias to megalomania, extreme sadism or masochism, severe depression or unwavering optimism, hallucinations, amnesia, ...
Similarly, as the dream-matter passes trough the body of the caster, a bit of its reality erodes. Slowly, the physical body will show more and more oddities and discontinuities. Examples of later stages are missing body parts (even missing connections between two fully functional body parts), boiling skin, more or less joints, supernatural abilities or disabilities (eg, a vampire's weakness to garlic), ...
Ultimately, either the mind or the body of the caster will give in. If the mind collapses to madness, the caster will become a monster, controlled by that same madness. (A bone file is generated, with a monster instead of a ghost.) The monstrous caster will haunt the place where it died, in a manner controlled by its madness. (eg, a megalomaniac will attack relentlessly, thinking himself invincible. An agoraphobic caster/monster will wait for prey in the smallest, most winding and cramped corridor it finds)
If the body erodes completely, the caster is cut loose from this world and sucked into limbo. From there, he has no choice but to endure, hoping another caster will one day open a portal nearby, trough which he can try to escape.
That's my idea about the whole dreams/magic thing.