Unlike most modern apps, Photoshop only has a single "undo". If you press command-z a second time, Photoshop will redo the change instead of continuing to undo. Not even Illustrator or InDesign works this way at this point.
To work around this issue, I customize my keyboard shortcuts to use "step backward" (command-z) and "step forward" (command-shift-z). For the most part this works great and I've been very happy with it.
But it brings up a new problem: if I change the selected layer, then press command-z ("step backward") then the layer change is undone as well. This is a constant frustration for me, and it seems there's no solution to get both the behaviors I want.
To me, there are two possible ways to fix this problem: • Adopt a standard undo/redo system with multiple levels of undo/redo • Make layer selection a separate step in the history
Personally I don't care that much, I'd be happy with either. But adopting a standard undo/redo system makes more sense to me, for consistency amongst the CS apps (as well as other apps). Perhaps it could be an option if there's disagreement over what works best for people.