When is an experimental variable good?

A experimental variable is good when you are trying to test an experiment.

A variable may be easily controlled, isolated, and measured.

For instance a difficult experimental variable may be a persons gender. You can control if a man or woman participates in your experiment, you may even easily identify which gender they are. But it may be difficult to determine if the results are due to inherent genetic differences, or social influence on their lives. that isn't to say it can't be used, it just is more complicated than a "good" variable.

A good variable could be something like Height, eye color, etc. that don't have that confounding aspect.

In summary -

The primary variable should be controllable, where you can directly manipulate it. The more controllable, the better.

The variable you measure, and the one you control, should be isolated. You should be able to distinguish it from other complicating factors.

The variable should be measurable. Some things are easy, such as height. Others not so easy, such as pain, or opinion. Developing tools to measure such variables are key.