I'm still new here (only just got approved last night) and I wanted to start posting immediately, but I had to build up my post count first. Nevertheless, this is my first attempt to post something useful to the community. I hope this isn't a repost (I searched and I couldn't find anything) or it's not a useless exploit that can't be replicated.
In Dragonblight, there's a Flight Point and town called Venomspite (near Naxx.) that offers several quests to fight the nearby Scarlet Onslaught. The only quest that is worthwhile taking and completing (for the purposes of this exploit) is Fresh Remounts offered by Hansel Bauer (I'll explain why later).
Once you've completed it, head back down to the outskirts of New Hearthglen and start killing mounted knights. Target the Knight, NOT the horse. I did this as a Mage, using Fireball/Pyroblast/Frostfire Bolt and Fire Blast. The Knight cannot dismount to engage you in combat, and thus, you can kill him with no threat to yourself. As long as you don't aggro the horse, the Knight has no way to reach you.
If you have previously completed Fresh Remounts, the Horse will turn friendly when you kill his rider and you can loot the body without any danger. If you haven't, it's not that important, since the horses (by themselves) have a very small aggro circle and it's possible to loot the body of a Knight without aggroing a horse.
I was getting (with rest state exp) about 1.4k a kill + silver + Frostweave cloth + the occasional green. I like it more than some of the Northrend AoE farm spots, since there's no danger and they're on a fairly fast respawn timer because of the quest.
Update
The best class to do this with is a ranged DPS, such as Mage/Hunter/Warlock/Shadow Priest
You could make it work with most other classes, except for melee only ones. Death Knights have a couple of ranged attacks (can somebody try Death-Gripping a rider?) and you can always turn to wands/bows/guns
Short Version:
1. Get to Venomspite
2. Complete "Fresh Remounts" (Optional)
3. Kill mounted Knights (not their horses)
4. Loot