Some combination of the above; I'll unvague that some:

If you're a heavy armor shaman:

To get maximal value out of your specialization (which may not be worth your while if it otherwise compromises what you're trying to do with your armor, there are trade-offs, etc.) you probably want to aim for pieces of armor that are actually armor, and are made of heavy materials like metal, stone, and sometimes bone.

If you're a light armor shaman, you probably want to avoid the stuff that heavy armor shamans like, and additionally you prefer lighter weight armors.

Some armor (including a lot but not all of the dragonscale stuff I checked) falls into a kind of limbo wherein it's not as heavy as a heavy armor shaman wants but it's not as light as a light armor shaman would like either.