For contradance music I don't like open guitar chords, hammered dulicimer, sympathetic strings, or the sustain pedal. Why would I hate on nyckelharpas? They're so cool! The problem is that in all of these instruments or playing styles strings are left to resonate on their own, giving an effect similar to reflected sound. For the most part your music will be the most fun to dance to if you time every sound you make to emphasize the offbeat, beat, or some rhythmic syncopation. Highly resonant instruments impede your ability to communicate rhythms to the dancers.

(While not working the same way, the drones on bagpipes play a similar role. I love the melodic chanter, played with careful ornamentation to communicate the rhythm, but for dancing the constant unvarying sound of the drones doesn't add much.)

So why do we have these resonant instruments, and what makes people want to play them that way for dancing? I think the biggest factor is that they're louder. When a guitarist stops playing bar chords and lets open strings ring out, the instrument gets bigger and more impressive sounding, but much of this is just a volume increase. With amplification we can make rhythmically muted chords just as loud.

It's also important to separate what sounds good for a single instrument played for listening and what sounds good with other instruments or for dancing. If your goal is to inspire your troops and strike fear into your enemies, the sound of the great highland pipes, complete with drones, works very well. If you want to provide a grand rich sound to support a singer, the piano's sustain pedal may be your friend. Aside from limited use as a texture effect, however, reverberent sounds aren't a good fit for dancing.