According to Wikipedia, being submissive means yielding to the expressed will of another person. A submissive (sub) shows this characteristic by his obedience, in private or public. Many of the articles on BDSM say that obedience is the gift from the sub to his Domme.
Another way a sub shows submissiveness is to wear the name of the Domme. This could be shown on the screen name ID in the chat room, on a collar, or on the body such as a tattoo. This identity with the Domme can be a very pleasing experience for both Domme and sub.
Many subs are given tasks to perform, essays to write etc. A good sub shows their eagerness to please by performing these tasks as soon as possible and by doing an excellent job (ie:good spelling, grammar, etc).
There are times when a sub is given a very difficult task, maybe even one which he had never done before or thinks is impossible. The absolute duty of the sub is to attempt to do the task to the very best of his ability. If for some reason, he cannot complete the task with 100% satisfaction of the Domme, he must be honest and tell the total truth about it, getting advice from the Domme as to how the task could be completed satisfactorily.
All of which bring us to the main characteristic of a good sub: honesty.
Honesty means being factual and correct within the relationship. It also means fidelity. No other characteristic is more important.
To sum it all up, a good sub is obedient, eager to please, completes tasks, and is above all, honest.