If you look at the history of the Icelandic horse breeding evaluations, there has always been a lot of discussion about how to judge spirit. In the 1980's the horses were judged on both willingness and disposition--two separate scores. And the weighting of what is now "spirit" has changed over the years also. At one point, it was weighted at around 17% and today at 9% .
The original designers of the breeding evals were specifically not looking to reward fear. Per Marit Jonson 1988 Judging Icelandic Breeding Horses: She quotes from a pre-1988 breeding standard:
"Willingness and self-propulsion is the most important quality in an Icelandic riding horse and is the foundation for all other riding qualities. Irrespective of an excellent predisposition for all the gaits, a horse will never achieve top notes unless it has the prerequisite willingness. The horse must push ahead willingly in all gaits, always a little bit faster than the tempo indicated by the rider, so that it is always pushing a little at the bit. The willingness will also show itself by the fact that the horse will go ahead without sticking to other horses, and that it will continue even when tired. The willingness may well approach the uncontrollable as long as it does not exceed this limit."
"This untamed will, this reservoir of power, which must not be confused with fear or nervousness [I added the bold], is what makes our small horses appear so big. If you have ever tried to cross an Icelandic desert on a tired horse, you will appreciate the enormous value of this strange gift."
She goes on to note that horses in Iceland are more "willing" than those bred in Europe. To score a 10 the horses should be "'Live volcanoes' with a large power reserve and indefatigable energy."
Disposition is defined as "It is obviously important for a horse to have a good character, to have the ability to learn, to be cheerful and courageous, docile and cooperative. The disposition is so critical that the value of its will to run is wholly dependent on the character of the horse. We do not, after all, want surly, nervous or stupid 'live volcanoes'".