The sound is the most important reason, followed by its appearance and
quality of workmanship, which is hopefully reflected in its reputation.
Measuring the sound can be tricky. I think responsiveness to any kind
of touch is important. I think a harp should have some kind of warmth
to it, and be even in character from top to bottom–no changes of
register like a voice or other instruments–all one voice. Then
consider price and availability. Some harps have become popular, but
it’s not based on real quality, it seems, but other more shallow
reasons, perhaps appearance, price, ease of purchase and newness, or
just lower price. Harps are also individual, and you can find gems and
clinkers anywhere. Make sure the materials used are acceptable. Harps
should be made of wood, traditionally speaking, with metal mechanism.
In America, spruce and maple are the best because they are rich and
strong, and bright and warm. Also consider how long you want it to last
and if you are going to be professional. If you are going to be
professional or hope to be, you need an elegant instrument of beautiful
design and sound, and sturdy manufacture. If you are a student who will
only play at home, you have a wider range of choices. If you are going
to study, but maybe not play when you are adult, then you can buy any
harp pretty much. Some require more maintenance than others. Talking to
a technician or a very experienced professional (offer to pay for their
time) is a good way to know about different brands, models and so on.
The sales people can guide you to the right size and match what you
decide you want with what is available.