World of Storm and Fury:
1. The magic skills of a character must be fully described in the profile. This includes how the magic works (in terms of effects) and how long the magic takes to prepare, as well as how the character came to posess magic (untrained natural adept, master scholar with many years of training, or something in between). The level of power of the magic ought to be commensurate with the background and age of the character.
2. Magic powers must fit in with the overall concept of a feudal world. (See #5 below for more implications of this rule.) The nations of World of Storm and Fury are, in the main, ruled by non-magic using aristocrats. In many nations, magic users are considered as mere servants, or the most powerful as scholars. Thus, the world-changing effects of magic are necessarily limited. Magic ought to have a key effect in a storyline, but not an overpowering one. And unless the magic user posesses significant combat skills (and a background to explain why the character is both powerful in magic and combat), the mage ought to either strike from hiding, or need significant protection from non-mage allies who are handy with sword and bow! The skill of the mage ought to have an effect as well. And time, environment and materials at hand to perform the magic should be key (it might be one thing to stand in an open field during a lightning storm and cast lightning bolts, it would be quite another to stand on a dry day before a charging regiment of heavy cavalry, trying to remain calm and compose the spell even as the ground shakes beneath oneself and the pointed lances grow quickly closer!).
3. A magic user must very clearly describe the casting and intended effect of the magic, in much the same way as 'an archer shoots an arrow' would be understood by the 'target' and the 'bystanders' in the scene. An arrow is shot, basically three things can happen once it leaves the bow. It can hit the target, it can hit something else, or it can miss everything entirely and land on the ground. And then another player would understand, if the arrow is lying on the ground, that his character could break it, pick it up and shoot it back (if he has a bow), or ignore it...At any rate, a magic casting ought to be that clear. Since almost everyone can picture a bowman shooting, but none of us have an exact idea what is involved in flinging magical icecicles, the description of what the mage is up to must necessarily be quite detailed, in order to allow the other players the level of understanding necessary regarding what their characters see and feel. (And as a corollary, the other players in the scene, out of respect for the writer, ought to acknowledge some effects from a well-written magic casting, rather than just ignoring the effects in their own posts.)
4. If the magic is just overly powerful, other players are apt to just ignore the effects (posting 'misses' so to speak). A reasonable level of magic effect, on the other hand, is interactive, other players ought to enjoy writing their characters with it. And so 'weaker' magic is actually more powerful, in the sense it is more likely to 'succeed.' Keep in mind the idea of freeform is not to have one's character 'overpower' the other characters in a scene, but rather to write with those players, in a collaborative story, with plot twists and turns no one writer would conceive of. Thus, for example, the lack of one key ingredient for a spell ought not to just be brushed aside by the magic-using character with an easy substitution of something else. Instead, the need to obtain this key material could itself form a significant part of the storyline. Or perhaps, something else is substituted for the missing ingredient, with unexpected results...The player ought to understand the latter two outcomes are far more interactive and supportive of a collaborative storyline than the mage merely continuing as though there were no difficulty.
5. Characters of course ought to use only IC knowledge--which includes what a medieval/feudal era mind could conceive of. An example would be the Roman Empire had the technology to build lightbulbs. They could blow glass, extrude wire, and draw vaccuum. But since they had no concept of electricity as a power source, they had no reason to make lightbulbs, and hence their mind could never have conceived of a lightbulb. A magic user can therefore only create objects and spells a pre-modern technology mind could have conceived of. See the Technology Rules for more specific information.