Nori is the dark green sheet used to make rolled forms of sushi. Made from algae (though most people think it's seaweed), it holds the rolls together but also adds the hard to describe, savory flavor the Japanese call umami. Until recently, I thought the nori's contribution was a minor component.
I was in a Korean market (Hankook in Sunnyvale, CA). There was a table in the back where a woman was packing sheets of nori in plastic envelopes. She had a saucer with samples. I tasted one and it was different. Much more flavorful than any nori I had ever tasted and with a lingering, fulfilling aftertaste. It looks different, too. Not as flat or as evenly cut as the packaged nori, it has a few holes. It's more delicate, requiring some care when rolling.
I bought the ones labeled as being lower salt and they were just salty enough, I thought. When we made sushi with it, the difference was remarkable. The umami accentuated everything, without overpowering it.
I'd never have thought nori would make a major difference.