Primary farming in World of Warcraft is new to The Burning Crusade expansion. It’s a great way to make solid, stable gold to get that epic mount or new armor you may need. Obviously, Throne of elements is the best place, but it usually has a lot of others that can slow down earnings. Scald is usually a bit better and is the best place to grow Primal Fire.
If you need Primal Manas Netherstorm this is the place to go. Especially outside area 52. Mana wrathes and wyrms have good drop rates. Or the ghosts of Mana around the ruins of the village of Kirin Var. Primal Air can be found throughout Nagrand and in Shadowmoon Valley. The mobs released by motes are usually quite scattered and difficult to farm.
The Nagrand Halaani Basin is one of the best places to get primeval water. Mobs are low-level and easy to farm. There are plenty of them and they are not immune to frost, making it the perfect place for Frost mages to grow whatever Primordial water they need.
There is a lake at the northern end of the Terrokar Forest that has a type of Mana fish that can drop Primal Manas. This used to be a good secret spot, but today there can be competition there.
Primal Earth and Life must come from the Gathering profession, but if your server supports it, you can farm the Swamp Lords north of the Swamp Rat Post in Zangar Marsh to gain Primal Life. And the various Earth elementals in Nagrand for Primal Earth. For Primal Shadow Farming, the Void appears in Nagrand or Hellfire.
Above all, growing Primals is a solid way to make gold. It is easy and pointless. Try to focus on the Primaries with a good value in the Auction House on your server.