Nielson's Model of User's Expertise
Plowing through research on navigation, just about everyone cites user expertise as a factor, regardless of the task being studied.
I'd like a better, more quantifiable, way to summarize a user's level of expertise in a persona. This could lead to generalizations about what types of interaction will work for certain types of user.
Jakob Nielson, in Usability Engineering (as cited here), divides computer users into six categories along three dimensions based on the user's experience: users with minimal computer experience and users with extensive computer experience for the dimension of knowledge about computers in general; novice users and expert users for the dimension of expertise in using the specific system; and user ignorant about the domain and users knowledgeable about the domain for the dimension of understanding of the task domain.