I really love many, many things about trail and ultra running. But, to be honest, I don't really enjoy the races as much as the training. I really love the feeling of leaving the house or trail head with some water and a little food and being able to be alone with my thoughts and nature for a few hours. If I had to choose a word, it would be "solitude".
I would also like to add a link to a great site for Trail and Ultra Running Resources.