PeccatorImpius answered:
October 12, 2010
For me, in theory, it is the course itself. In practice, however, it is often the location (i.e. convenience). I have a desire to run the Avenue of the Giants Marathon and the Catalina Marathon, both because of the uniqueness of their courses. However, many of the races I have chosen in the past were because they were close to my house and convenient to get to by car. Hopefully one day, desire will win out over convenience.