Right now at 69 I'm getting 14m exp every 2-3 minutes. Don't really get any gold or items.
Start a quest in act 1:1 Rumford at the Gate on Normal
Talk to Rumford, and kill the Wretched Mother
After, go and kill 2 of the 3 Wretched Mothers for the secondary quest that pops up, DO NOT KILL THE THIRD, IT'S IMPORTANT.
After killing 2, leave the game. Change your difficulty to torment 6, and resume the game.
Pull the wretched mother that is right outside town, towards the gate, you need to get her inside the town, and she auto dies, completing the quest.
I've done it from level 1 on my Wizard, only taken me about 2 hours to get to almost 70 now. 0 deaths.
I could be wrong and this could just be a guide, but I'm not positive if quests are supposed to reset upon changing difficulty when leaving the game. Either way, a neat little trick that works pretty well.