A while back I polled the blogosphere about what y'all thought the most desirable trait of a sensei was. With 15 voting, the majority (60%) said that a sensei should primarily be a great technician. The rest of the votes were spread out across the answers with a little bit more emphasis being on spiritual leader and workout partner. Several people added other responses, including leader, mentor, expert, teacher, 'someone who can guide me past self-limitation' and 'all except spiritual leader.' From experience, I have found that I have been called into all of the above roles.
Personally, the traits I desire most in my sensei fall predominantly in the great technician and workout partner categories. I recall my Japanese language teacher in college, who was also a martial artist, telling us that the word sensei has connotations similar to big brother/sister or even trail guide. The sensei is someone who has been on the path for longer than you and who is still on the path, so they can act as a partner/leader. Not someone who says, "go that way," but rather someone who says, "follow me this way." In some traditional martial arts, this trail guide role seems to be assigned to or assumed by the sempai (more senior students who are not the sensei).
I recall my first martial arts instructor, Pat Little, telling us that, "those students that don't eventually surpass the teacher, fail him." I don't know if I've surpassed him, or if I ever will, but I think that was a pretty darn good motivator for me. Along the same lines, perhaps the best advice on learning and teaching martial arts that I've ever gotten came from Dr. John Usher, when he said that in order to ever become more than mediore as a student, you have to progress to the point that you can identify and diagnose your own faults and create your own plan to improve them. You have to figure out for yourself what you need to learn and how to best move toward that. In order to surpass your sensei you have to figure out how to become your own sensei. Usher is not only a great technician and workout partner, but he is gifted at creating these self-motivated learners.
For a good resource on self-directed learning, check out this wealth of information.