Selections: the uses and effects of different brushes, the concept and use of feathering.
The diffeence between brightness/contrast, curves, levels and histogram adjustment as a means of processing your shots, and when to use each, and when to use more than one.
Masks - how they function, the different ways in which they can function, how to control the artefacts left at their edges, and at the overlaps if you often invert a mask to use again.
To my mind those are the really useful things (and they apply across most decent image editors). Sharpening, noise control, colour control, blurring, the thechnicalities of compression when saving, re-sizing strategies, methods of conversion to black and white, saturation control more generally, are all things that you should have a thorough understanding of too - only I would consider that a given: how anyone can hope to use these programs effectively without knowing exactly what those controls are doing in terms of the pixels in their image is beyond me.
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