Act 1 Cathedral Nightmare Farming Guide
The Cathedral in Act 1 is a great farming location for all difficulties because I feel it is an introduction into what you should be expecting for that difficulty. If you can't get far past act 1 falling back onto Cathedral is a great idea as you'll have tons of mobs to kill and a ton of chances at loot which could help you with those pesky later acts. The only real downside of farming Cathedral is it drops loot from your current difficulty as well as loot from the difficulty you just came from. Although, if you're level 60 and farming once you get full Nephalem Valor you will mostly just be getting loot from the high end of Cathedral's level range.
To start a Cathedral farming run select the Reign of the Black King quest from Act 1. If you would like to get full Nephalem Valor before even entering the Cathedral choose the "A Shattered Crown" quest and clear each of the three tombs as you search for the crown.
Common Nightmare Cathedral Loot Table
Page of Jewelcrafting
Page of Blacksmithing