Help contribute to this thread to help out people who don't know little things in the game that make it a whole lot more fun / entertaining.

My tips:

1. Turn off other people's achievements

While in a game and someone you're questing with gets an achievement, (Or people on your friends list) you are notified as well. Sometimes this can be annoying so you can turn this feature off. TO change this go to [ Options -> Social ] and in the list on the left, uncheck "Friend Earns an Achievement"

2. How to swear

Whenever you use a swear word in game, it will block it out with random symbols by default. To change this go to [ Options -> Social ] and at the bottom right uncheck "Mature Language Filter"

3. Have any skill you want on your action bar (Elective Mode)

To enable MUCH more customization of skills, turn on elective mode. [ Options -> Gameplay ] At the bottom right, check "Elective Mode"

4. Always see how much damage you are dealing.

Instead of only seeing the damage you do when you get a critical hit, you can see how much damage each hit deals. Enable this by going to [ Options -> Gameplay ] and check "Display Damage Numbers"

5. Hidden Merchants

Some merchants are not in town, but random houses along the way of your quests. Many of these merchants carry rares and decent magic items. Make sure to check them!

6. Auto Equip

To have items you pick up auto-equip (Unless you're wearing something in that slot already) go to [ Options -> Gameplay ] and check "Auto-equip Items" on the right list.

7. Press "G" to throw your banner down anywhere in the game.