In order of importance
1.) spins
2.) actual instrument time
3.) a proper working understanding of how the engine operates, mechanics of the accelerator, primer, carburetor, injectors, etc.
4.) buying your own fuel at a self serve pump
I do believe those are skills and PPL should know. How many accidents are caused by spins and loss of control after VMC into IMC?
I also believe that a lot of inappropriate engine management technique comes from just not understanding how the engine operates. And when I say "understand" I mean beyond the basic four stroke process... I want to know what is happening in the engine when I pump the throttle on start, when I prime, when I apply carb heat, when I change the mixture. What is and why does manifold pressure change, how does it relate to power? What does the CHT and EGT tell you, why do they go up and down depending on mixture?
**As a non-IR PPL I consider myself very proficient on instruments, but my first actual IMC with a CFI in the Cirrus was a fun, but very unique experience, even with the fancy glass. I can only imagine getting IMC alone in a steam gauge 172 coming back tired on a XC.
As a PPL you should be able to handle safely any situation you can reasonably get yourself into. Do you need full IR and commercial training, no, but a spin and IMC into VMC, or stuck on top, can catch and trap a lot of people and you should be able to fly a perfectly good plane home without crashing it