This could easily be a list of things people take for granted.
That is exactly why i started this!
My list would also include the scripting ability. I LOVE scripting and with coding knowledge it can be soooo powerful.
I like the brush variance as it adds so much flexibility.
Ability to save workspace WITH the images open!
The history palette: great to undo many steps at once instead of going ctr-z 20 times.
Customizable workspace also. You can create your own menu with the tools and commands you use most instead of retrieving them with multiple clicks.
Binding scripts for even faster access.
TUBES, and mostly the directional ones. They are unique to PSP and can be sooooo great.
I`ll name some more later.