Some things people don't know and probably should. I am slowly adding to this. Feel free to comment on little things.
1. You can hit ALT to open the menu that is hidden to go into options, change character, etc.
2. You can buy, drop, give, sell, etc, max stacks of items by holding down shift. For buying and selling, you can also use CTRL to buy 5/sell 5 of the items. Similar to the above, while in a bank, use SHIFT to deposit all held gold or withdraw 1k. CTRL will withdraw a larger amount (10k).
3. Holding down SHIFT and using your scroll mouse wheel will cycle through the attack icons to the right. You can also use CTRL+1 through CTRL+5 for the attack icons, and CTRL+8 through CTRL+0 to scroll through the item icons.
4. You can release a pet/spawn without having to disband from your party. Simply use /recall to do so. (Next update will show you this in game directly now so shouldn't be such a hidden thing now.)
5. AD is average damage, and is the average damage your character will do with a weapon. AP is average power, it is the base min/max without enchantments ((MIN+MAX)/2) and can be used to quickly check if an item is better without equipping it. This is not always the case though right now as some classes may get certain bonuses with weapons.
6. Brilliant enchantments can be scribed by an Enchanter. (Level 25).
7. Embers work like divinities. If you have a Fire Ember (3%) equipping it on a weapon will give you more damage to night divinity. Equipping it to armor will give you more resistance to night divinity. Think of it like player divinities -- If you have 'XXXXX' divinity, what bonuses to damage/resistance would you get?
I.E. Being ice divinity means you get a damage bonus against fire divinity, and a damage resistance against fire divinity. The ice ember will do the same as an example.
8. /set is a client side command and you can use it to set macros through command line!
9. Holding down SHIFT and clicking Q through T will change your chat tab without having to mouse click.
10. You can lock the window with SHIFT+L if you don't have a scroll-lock key. This doesn't prevent buffer clearing still so be warned.
11. You can quick withdraw and deposit using the 'middle mouse click' OR 'ctrl+right mouse click' to/from your vault. This will occur on the window vault that is CURRENTLY OPEN, otherwise it will default to the global vault.
12. /togglegive will prevent you from receiving items from non-party members.
13. There is a friends list and a secondary text window for text. Check it out in the ALT>WINDOWS area.
14. Secondary text chat window and main window can be filtered somewhat. Go to main chat tab and right click it. Here you can disable sounds too. Secondary text window can be filtered in a similar fashion.
15. You can use the numpad keys to move (check numlock if you can't move with these), use CTRL+Arrows, click the arrows on the minimap window, or click a square away from you on the minimap window.
16. Clicking the stat button a second time will reveal some hidden information.
17. Have dual or more monitors and constantly move the Ember Online window between them? Find it annoying that the position is saved based on where the window was last? (Specially if it opens up on a different monitor!) You'll probably want to disable the 'Save Window Positions' in the options menu. This will default the windows to be centered when opened up.
+++Adding more as I see+++