Is there a clear-cut difference between dissatisfied and unsatisfied?
The feeling connected with a lack of satisfaction can have two different components:
A sense of incompleteness, which leaves one feeling unsatisfied
A sense of wrongness, which leaves one feeling dissatisfied
Naturally, both senses may overlap in certain situations. Take the following example:
John's father was unsatisfied by the principal's explanation of his son's injuries.
[He suspected that some important details were missing from the principal's explanation.]
John's father was dissatisfied with the principal's explanation of his son's injuries.
[He suspected that he was being lied to.]
If I was John's father in this situation, I would most likely choose to describe my feeling as dissatisfaction, which would better reflect my state of active displeasure, and would also encompass both the incompleteness and the wrongness I had perceived in the principal's explanation.
Had I merely felt that some information was missing, the result would have been a lack of satisfaction.
The big difference between the two words is that dissatisfied is only used of people. That is, a person can be dissatisfied by the amount of food they receive; but their hunger would be unsatisfied. You can use dissatisfied in places where you could use disappointed. When some requirement is not met, we can say that requirement is unsatisfied (but we would never say it's dissatisfied).
A more subtle and semantic difference is that unsatisfied often implies that a greater amount of something is needed, whereas dissatisfied tends to imply that the quality of something was poor.