One of the things that has made the Hero system such an enduring favorite is the fact that it is a very balanced game. There are no "absolute-kill" powers and no impenetrable defences.
However, this balance tends to come apart in genres other than the superheroic "anything goes" type game, so the GM needs to be careful when setting up his game world. It is very common to see games which have absolute point limits, or powers which are specifically forbidden. While this of course is the GM's right, and one way to establish a game-specific feel, it often leads to imbalance as powers which are restricted are often needed to offset other powers. Absolute limits (unless they are high) tend to push characters towards a generic middle ground since it reduces the benefit in specialisation. This doesn't mean it can't or shouldn't be done - merely that caution should be exercised. And all that goes double for games in fantasy setting.
Here's a sad story to illustrate what I mean (it is by no means the only example I have observed). I played in a game a while ago with a group of talented and fun players - and a very good GM. One of the characters was an Earth mage, with a variety of spells related to Earth magic. In the course of the game, we had to rescue a captive from the lair of the traditional evil mage. The first few encounters with his patrols went easily enough, in part because the Earth mage entangled many of the foe by making the earth turn to quicksand under their feet. But when we got to the castle of the evil mage, things started to fall apart quickly. Using her "Earth vision" spell the Mage detected a dungeon complex beneath the castle and quickly opened a passage by which the party could proceed directly to the dungeons and spring the captive. We were able to loot the treasury too and then escape, with hardly a fight. The GM was not too happy, seeing his carefully planned adventure - which was supposed to have occupied hours of play - demolished in minutes.
The moral of the story? Think about how magic will affect the kind of game you want to run. If it is in a heroic setting, where most of the players are mere mortals - albeit talented ones - then magic can tip the balance easily, because it provides its users powers that no-one else in the game has, and which can be very powerful even at low points levels. Remember to design your game with the potential powers of your players in mind. Interestingly, in my experience, it has not been devastsing combat spells that cause the most problems, but spells which allow unusual movement powers (tunnelling, flight, teleport) or unusual powers of perception (clairsentience, in particular).
For some suggestions on designing magic systems that work, you should return to the page on designing magical worlds.