I'm a Alchemist and Herbalist. In my down time you barely see me waiting around in Orgrimmar or Shattrath City. In that time I go farming often for herbs to use for Alchemy or my character needs. But in Shadowmoon Valley Nightmare Vine is the herb of the zone and its really nice to fly around doing donuts in the whole area looking for this herb as well as Felweed/Motes of Life. Nightmare Seed is gathered from picking Nightmare Vine. You can also be poisoned when picking this herb!
I have Cartographer and that really helps to spot herbs nodes you have farmed. When you herb pick Nightmare Vine you have a chance and its not a big one of getting Nightmare Seed with it. Nightmare Seed is really nice to get as its really nice to use for Tanking. Its also not a Soulbound item and if you ever see them on AH they also cost a bit. I did donuts around Shadowmoon Valley for 2 hours farming herbs I need. Most I got was 50 Nightmare Vines and 5 seeds for the effort. That's the most seeds I've ever collected at one time. However as a Tank myself the 2000 extra health for 30 sec is a whopper when your tanking a hard fight and need that extra edge. Nightmare Seed can work wonders if you can get your hands on it with Herbalism or on AH with the low drop rate.