The smooth stroke tool. Next up, the line, perspective and symmetry tools are invaluable and I can't believe other softwares don't feature them.
There are certain things it lacks which makes me resort to Photoshop more often than I should. The biggest problem is the lack of non-destructive editing tools such as (in order) layer masks, clipping masks and adjustment layers. Such basic fundamental tools that are completely missing.
The smooth stroke tool is amazing, but can be difficult to get perfect sometimes. Being able to create a path, adjust the path and then draw over it with a tablet similar to the line tool would be incredible.
Keyboard commands can't seem to be adjusted. Why? Comon.
If those changes were made, I'd never use Adobe Photoshop for rendering/sketching again. And that's coming from someone with 16+ years of experience with Photoshop.
It's not a substitute for Adobe Photoshop, yet. Make sure you still have ample Photoshop experience.
Concept visualization and rendering. Being able to convincingly render forms before spending time in CAD or 3d modeling.