I know its odd that this is my first reply but I love photoshop so I couldn't help it
I have been using photoshop since version 7, and I am only 18 myself, and am currently using CS3 suite, and I have found the most versatile tool to be the ability to apply image adjustments in the form of a layer, aka Adjustment layers,
I do allot of client work and Adjustment layers allow me to show them the difference between certain things quickly and easily,
Also I love the pen tool, the ability to make any shape imaginable and retain some vector attributes in a heavily raster program