Eddieee said:
If you beat a difficulty, can you go back to it?
Sorry for sounding so nooby XD.
Yup, you can go back. Every time you enter a different difficulty than you last played with that character, or join a different multiplayer game, you get new maps. However, any quests you completed will remain completed (no getting the Izual skill point reward over and over, for example), and you keep all your waypoints that you earned. This only applies to stuff you did in that difficulty, so if you complete Normal, doing all quests and getting all waypoints, you still will have none of them done when you first get to Nightmare.
If you're in Nightmare/Hell, and are planning on going back to Normal/Nightmare, remember the maps will change twice- you'll get new ones for going back to the easier mode, then new ones again when you return to the harder mode.
Like Wildjinn said, you might want to hold off on getting the quest reward for some quests. The Imbue quest will give you a randomly-generated rare item of the base type you provide to Charsi, but the level of magical mods is limited by your character level, so you might want to save it for high levels. Another quest you should probably hold off on is the socket quest, as it is best used on items you'll be keeping for some time, not discarding once you gain a couple more levels.