[This was part of a post I'm currently working on, and it discusses the balance I believe must be maintained in the magical systems utilized in the fantasy genre. I felt it could stand on its own, so here it is.]
Many fantasy writers start their book or series with a weak hero and powerful foes, and the "resolve" is that the hero is the strong one, and the villian the weaker one. To me, neither the hero nor the villian should clearly have the magical-upperhand. A better writer will employ non-cookie cutter villians; the resolve will often involve the villian being misunderstood and perhaps virtuous, or the villian was misguided and repents; or the hero is as guilty as the villian; etc.
Overall, there must exist a plausible understanding of what motivates the hero and the villian to use their magics. The villian shouldn't be depicted as a ruthless, homicidal miscreant, wishing harm and death upon others solely for its entertainment value. The hero shouldn't be a selfless, altruistic "friar" who is fighting the battles, and constantly putting himself in mortal danger solely for the sake of others. With believable characters, your mind can more easily suspend the disbelief required for magical tales, or any other fiction for that matter.
The second element of balanced magic is that there are no major magical suprises in the sequels. This means that once a system of magic is explored and its possibilities are discovered, you simply cannot write sequels utilizing the same universe of magic, but which also burst the envelope of limitations that the previous stories and struggles were contained within. Simply put, if the magic used in the first book was only a subtle magic, fantasy writers can't use cheap parlor tricks on readers to accept the fact that now the magic is now a hundredfold more deadly, explosive, fiery, etc. Its an insult and disservice to readers who respected the earlier set magical guidelines, and whom invested time to understood the challenges that the characters faced and the actions they undertook within those limitations.