I've worked with several (just trail riding) and I wouldn't term them as "nervous", but they were "busy" all the time. Always looking, prancing, kinda flashy...didn't care to be tied for very long. Generally just not "relaxed" horses. None that I worked with were headstrong or stubborn but seemed to like to learn. It almost seemed like they wanted to examine everything....like crossing a swift creek...a quarter horse would just stop and stand stiff legged until it crossed...the arabians would walk up/down along the edge until it found the angle it wanted to cross at, never stopped moving or balked just had to check everything out.
Just my limited experience, but it seemed to be common with the ones I worked with.
