Considering how variable it can be from person to person, I don't think there's any single answer out there. You could interpret the DSM-IV criteria, applying the listed symptoms to a less severe pattern, or you could omit some and not have others.
I'm pretty sure the staple, shared feature of Asperger's is poor social interactions from a young age, without speech or other obvious delays in development. Mild Asperger's could mean mild social impairment, then, but specifics would be down to the individual. And then there are causes of the behaviour and whether or not it's Asperger's, true autism, or just a bad childhood.