Keyboard shortcuts. If you are going to do anyhting in volume you must know the shortcuts.
I say the thing that is most foreign, and therefore most important, to new users is the concept of layers and masking. Next, I say adjustment layers and other NON-destructive editing. Last Learn how to save for each different medium.
A BIG problem I see around here is lack of moderation in Post Processing. Things like oversharpening, blind action application, etc...
Maybe I'm getting too ahead of the topic. My number one is learn the program, it's workspace, where everything is located(even if you don't know how to use it). If you know where something is and how to access it then you can at least get by.




