Ability to build trails that user will continue to use over and over. User desired experience is. Involvement of the user is vital. User's experience and to get them involved. Using visitor information to form volunteer groups where they currently don't exsist or are to few and unorgainized to be effective.
Users have the most to gain but are under utilized.
Variety of tools and methods for accomplishing trail work. If I do not understand what can or cannot be accomplished using, for example, blasting methods, I can't employ a full range of options to solve a given trail related problem. Methods of pinning barriers, guide structures, or support structures. Understanding of trail development history. Psychology of users.