What Kinds of Problems?

What sorts of problems / needs / objectives should your characters have?

To some extent, the answer to this question is genre-driven.  In a mystery, the detective needs to find out who done it.  In a thriller, the protagonist must stop the bomb from going off.  In a romance, the protagonist's big concern must be to overcome the obstacles to love.  In middle reader and young adult books, the protagonist must (and other key characters may) have needs that are universally comprehensible to children: finding one's place in society, discovering one's identity, relating to adults / authority, first encounters with the opposite sex, etc.

Beyond that, I offer some observations.  Needs are stronger if they are universally or nearly-universally shared or shareable.  They are stronger if they are concrete and urgent.  Jill must recover the vaccine and deliver it to the hospital in time or her mother will die is stronger than Bob would like to win the flower show this year, though there are stories in which the desire to win the flower show would be a fine motivation, especially if Bob needs to avenge the humiliating loss last year to a romantic rival, or has to win the flower show to keep his job.  If needs are abstract (e.g., a fear of mortality), try to root them in concrete common experience (e.g., the loss of a loved one).

Conflict is generated by multiple characters in your story having pressing, urgent needs that drive them to fight for conflicting objectives.  Conflict is crucial because conflict generates interest -- and interest is how you get people to read your book.