Hey guys, so as I was running through Nightmare on my 35 WD, I found a pretty nice way to get experience.
The initial setup takes roughly 5-10 minutes, depending on how fast you can kill/sneak past mobs.
Also, there are three things to note. They aren't "required" but they will speed up your exp gain per hour massively
1) Good AoE. As a WD, I went full AoE, and completely trashed these mobs. It's essential to kill them fast, as we're not doing any quests, just grinding.
2) Monster Kills grant +X experience. You want this affix on nearly all your gear, as you are fighting a LOT of mobs, therefore the more exp you get per mob = the better.
3) In my opinion, this one is nearly required. Put a good RUBY in your helm. I am using a Flawless Square Ruby in my helm, which gives 19% more exp per monster kill.
Now that you're setup, on to the steps.
1) Start on Act 1 quest 8 part 2 (Khazra Barricade)
2) Take the WP to Highlands Crossing
3) Run up to the Khazra Barricade (you have a yellow arrow guiding you)
4) Fight your way through all the monsters
5) Go through the gate at the top
6) Follow the straight road until you get to the stairs.
7) Fight your way up the stairs. Once you get off the stairs, go left towards Leoric's Courtyard
You'll get a checkpoint once you hit the courtyard, this is what we want.
9) Run into the manor, clear the inital group (TONS of mobs). (Do NOT go into the next room, where the quest starts. If you complete the quest, you get 21k exp [NM values], but you have to start from step 1, so not worth it.
10) Leave the game, click resume game, and start from step 9.
As a level 37 WD, I was getting a clear (this included killing them, leaving, and then joining a new game) in 30 seconds, and getting 37034 experience per minute. Considering I need 260k to level, that's only about 7-8 minutes to level!
EDIT: Two more things to note.
1) There is sometimes a shrine right where you spawn (or close by) and if you're lucky, it can be an exp shrine, so watch out for those.
2) Also, there is a rare spawn mob in the Manor every once in awhile, and he nets a decent amount of exp as well.