Before I get started, all credit goes to
Markco @ justmytwocopper.blogspot.com :wave:
Alright as you all must know there are achievements for old world instances such as Zul'Farrak, The Deadmines, Marauden, so on and so forth. These are easy achievement points you can get done in a couple of hours.
If you've checked the Auction House recently you'd know that some of the old world cloth like Rune Cloth, Mageweave,
Essences of Undeath sell for insane prices now for leveling proffesions.
Instead of just running past every single mob in the instace that you won't actually aggro at level 80, kill them, and LOOT THEM! You can actually make good money from (the now rare) essences, cloth, and greens, and raw materials you can get out of these instances.
tl;dr Players going back for old world dungeon cheevs are missing out on a good chunk of gold (700g+) if they pass up on looting these mobs.
Please remember this doesn't compare to farming in Northrend.
This is just a quick, fast, easy way of making a chunk of change very, very easily.