I knew that I wanted both spells and rituals in Destined, and I also knew that I wanted the distinction between them to make sense. My solution was to make rituals into more formal versions of spells. Spells are quick, flexible... and dangerous. Rituals are slow, more static, more reliable, and far more safe.
How are spells dangerous? I'd discussed how to limit magic use here before. I don't think I explained what I settled on. The first thing to note is that to cast a spell very effectively, you need a number of degrees of success which can be used to do things like increase the spell's duration, range, damage, etc. On the flip side, spells have a penalty to their casting roll based on their power level. Spellcasters will be scrounging for bonuses (there are a variety of ways to get these) and will need to manage them wisely. They can get by on casting spells without bonuses, but those spells will tend to be much less effective... and they are far more likely to fail altogether.
When a spellcaster fails a roll to cast a spell, he has a choice: he may accept the negative shifts on his failed roll as a penalty on all his future spellcasting rolls for the day (until he gets at least four hours of sleep) or he can suffer a penalty such as fatigue or damage that is based upon his degree of failure. If the spellcaster just barely fails, then he doesn't suffer either of these problems. Instead, the spell misfires. In general, this means both that the spell has an effect that is slightly less useful than that of a one-success casting and that it has some unintended (and unwanted) side effects. For example, a misfire on a simple Light spell might result in one of the caster's fingers glowing in flashing colors that fade over the course of a few days. If a spell misfire occurs and the GM sees an opportunity for a spectacular spell failure, he may offer the spellcaster a fate point (which the spellcaster can later use for a bonus). If it is accepted, the GM may describe the misfire any way she wishes. In general, established rituals do not misfire. This is one of the benefits of rituals, but it also accounts for their cost.
It is possible for a spell to be cast as if it were a ritual. You could perform a ceremony that ends with a fireball... and no chance of misfire or backlash. There are other costs involved in this, though.
I'm actually really happy with my ritual system. Nonspellcasters can, potentially, learn rituals. In some ways, they can even be better at using rituals than spellcasters (though it is more of an investment for them to learn how to use them in the first place). The primary factor here is that identification and learning rituals depends upon the Lore skill rather than the Spellcasting skill. In addition, many (particularly more powerful) rituals must be performed at auspicious times, and the caster must calculate the next time they will be able to cast it. For a rare and potent ritual, it might be a number of years before it can be next used effectively. If a ritual has an
auspicious time listed, this is the length of time until it can be cast. The caster can attempt
to calculate a sooner time; this, too, is a Lore roll.