parsimony

In general, the system intends to produce outputs with the smallest amount of thought from the reader. In short, 'parsimony'. I'll illustrate with a few examples.

A good example is the Lending Hand spell from the Fate sphere, which allows the mage to give someone a +1 Bonus to any Skill, so long as the mage has a higher level of that Skill than the target. The spell targets others, but it doesn't demand that the system have any explanation of 'target self' or 'target another person'. The spell simply doesn't work on oneself, because you don't have any skill that is higher than itself.

The spell also remains thematically consistent with a given priest's calling. A priest of war may gain the Combat or Projectiles Skills, and be able to raise others' Skills, while a priest focussed on learning may be able to boost others' rolls in Academics for a while. However, we don't need five spells here for five different deities. The single spell has the same effect as having multiple, bespoke spells, but the players only need to bother reading and recording one spell.

The spell enables people to create magical items, so the system has no need of a 'hoe of gardening', and 'gloves of theft' - almost every conceivable magical item which makes people better at something naturally falls out of the same spell.

A mediocre example of parsimony would be the Aldaron spell Freezing Touch. While it has multiple effects - allowing casters to freeze a river and walk across, or freezing an enemy - these come with the cost of explaining how each effect works individually. It would have been better if the spell could describe an individual effect which could be used in different ways.

An egregious example of this would be the Eshu spell to summon a horse from Changeling: The Dreaming. The book already had a developed magic system, but at one point in a supplement it was mentioned in passing that Eshu changelings typically know a ritual which summons horses within certain areas. This was a wholly new notion of magic, which had to be simply memorized. It didn't fit with anything before, which makes it more work for the players.

So in general, the idea is that a lot of thought is best employed in making systems which are both simple and expressive. Every new concept, such as the difference between an 'instant' spell and a 'continuous' spell, should be used and reused to get the most bang-per-buck. Every new idea in the book should be viewed as a heavy cost, and anyone reading it should expect a lot in return for their attention.