Sticky Targeting
When this is turned on in the options, you will not deselect your current target when you click on empty

Autoloot
Use Shift-Right Click on a corpse or chest to autoloot it. This will NOT autoloot bind-on-acquire items.

Equipping Items
You can equip items by right-clicking them from your inventory. This will also place items in the bank.

Tab
You can cycle through targets with the Tab key. If you do that then manually select a different target using another method, then quickly hit Tab again, your last Tab-selected target will be re-selected and you can continue Tab-selecting where you left off. Clearing your target will restart the Tab selection process.

Swimming
If you run out of air, you will die. There are various items and even potions that allow you to breathe under water. You can also use the jump key to swim upwards while in the water. You can even jump up out of the water using the jump key.
To dive, you have to tilt your direction downwards a certain amount with the right mouse button.

Logout
You cannot logout while in combat.

Alt Tab (minimize program)
If you alt-tab away from the game, your character stops moving unless you are on auto-run.

Alt-F4 (close program)
Alt-F4 can be used to close World of Warcraft, however, it will not instantly log players out of the world.

Different Characters
Add-ons, Macros and Keybindings can now be saved on a per character basis, so you can have different characters load different add-ons/macros/keybindings.

Tips
In the initial loading screen there will be a tip displayed. This can be turned off from the interface options menu.