Width vs. Breadth is a principle used to evaluate the acceptability of a proposed power.
So you want to design a power. You want it to do things. Lots of cool things.
How do you decide how much is too much?
The Width vs. Breadth theory of power design offers a quick rule of thumb:
The more things a power can do, the less effectively it should be able to do each of them. []
In other words, we do allow powers that could (potentially) be applied to a lot of different applications. Telekinesis could, for example, be used to make a shield to protect yourself, increase your physical strength, or to fly. So, clever you, you decide to get all excited about this. "This sounds great!" you say. "If I get that, I can be as good as anyone else with any of those three abilities!"
Nope, sorry, not how it works. Telekinesis operates on a simple principle: thought energy transformed into kinetic energy. There are obviously quite a few things you could do with that! But because it's so broad, by convention, we assign it's individual parts a lesser power. In other words, the average telekinetic can't fly as fast as the average person who just has flight, nor can their shield themselves as effectively as the average person with force fields. They can do a little of lots of things, but they'll be beaten in an individual category by a specialist. This principle is behind the banning of Omnikinesis; on its basis, technically every character has power over everything, but what they can do with that power is so limited it's not actually a power at all!
That's not to say there can't be exceptions. An extremely powerful telekinetic might be able to fly faster than an extremely weak flyer, but for the most part this is rare. Obviously, the status of what constitutes the "average" strength of any given power on the RP is something of a moving average, and staff looks to scientific principles, previously approved powers, and their goals for the site when deciding if a particular power is acceptable. Just understand that you will probably not be getting something that is really several full strength powers in one. That's not what we're going for here. It's about making characters, not about inventing a tricky power that can do anything.
On a related topic, while staff will typically tell you if you are asking for a power at a level above what is acceptable, they will typically not tell you that you could ask for a higher level and power and be accepted. For this reason, some members have taken to deliberately asking for powers stronger than they believe to be acceptable, expecting a reduction to be demanded in order to assure the maximum potential for that power is achieved. The wiki team neither officially approves nor disapproves of this practice.