Regular Cast Members are entitled to one Power without having to define a weakness. For each additional Power (and multiple powers are popular) they would either need to spend to requisite Plops or take a weakness.
Broadly defined, a Weakness is anything that reduces a character’s options, or impairs their ability to function normally.
In theory, an RCM's Weakness should be roughly equivalent to his Power. In practice this is really hard to do, but we can at least get somewhat proportional. The Weakness should be persistent and foreseeably permanent. There should remain the threat that it could become an issue every episode.
The Weakness does not have to have any direct relation to the power it balances, or any other power. It is subject only to Rule 3.
Below are some examples of what kind of weakness can be used to balance specific levels of Power.
Normal:
Allergies, phobias, odious or annoying personal habits, strong moral code that restricts tactical or social options, minor disability such as bad back, or nearsighted, common substance dependency (such as alcohol).
Extraordinary:
Stat at -3 or non-existent; A status of Good or Evil; Permanent severe disability (blind, deaf, crippled); uncommon substance dependency (such as heroin); serious psychiatric condition; Inability to consistently use and/or control the power; Strange or unusual appearance (by any standard).
Supernatural or Fantastic:
Rare substance dependency (such as plutonium); Moral code that causes emotional damage if violated; Obvious inhuman appearance; Use of power causes threat to self or others; Geographic or temperature restrictions; Requires special sort of energy to recover.
Two Normal weaknesses can equal an Extraordinary Weakness. Two Extraordinary weaknesses and/or four Normal weaknesses can equal a Supernatural/Fantastic weakness.
The point of requiring a Weakness is not to create any real play balance. We don't actually care. The Point is to provide an aspect of the character around which plot complications can metastasize. Consider that foremost when negotiating these things. A creative, imaginative weakness should score higher than a dull but crippling one. We don't want the character to be crippled. We want the character to be interesting.
If the Power in question is a Special Status - we take it on faith that the Weakness has been built into that.
"Requires Magic" is not, of itself, considered a weakness.