Ahhh, checklists.
Amplifying what has already been said, the first port-of-call must be the manufacturers flight manual, if there is one. Everything else, especially add-ons, requires a bit of cynicism. Of course, our new PPL student hasn't the experience to question such things and that is where the rot sets in.
Secondly, the 'why' and the 'what'. I have the great pleasure of flying with 200hr CPLs who have just finished the training gravy train. They're quite good at procedures until I ask them 'why/what?'. "Why do we switch that switch at that point? What does that switch do? What are you going to do if that switch doesn't work?" Many are lost for words at this point because they haven't read the book. They've learnt their checks and not bothered to get as far as Chapters 7 & 8 of the flight manual. Bearing in mind these chaps are now getting paid to fly, I occasionally go as far as getting them to explain every action before they do it.
Lastly, the lack of appetite to improve or fully understand. Back to my 200hr CPLs. We operate in a MEP (twin) environment and consequently do not have the luxury of aircraft that comply with all the V1, V2, Balanced Field protections that apply to your average Airbus. So, we sit at the holding point and Biggles Jr delivers a comprehensive departure brief, stipulating Vr speeds and actions before/after. The brief almost always includes an ".......engine failure before Vr I will abort the T/O and we will stop on the runway"; I highlight that Vr will appear somewhere about 100m before the end of the runway and there is no way will will ever successfully stop under such circumstances. The important bit is that I then send Biggles Jr away to do a bit of homework but, you guessed it, he regurgitates exactly the same brief next time we fly.
We have become lazy in aviation, something that goes against my particular grain; It is now acceptable to miss the centreline on landing. I was encouraged from the outset to seek constant improvement, understand how things worked and why we did something with an aircraft. We now have a tendency to look upon aviation as a number of hurdles and not a continuous journey. There is little appetite for betterment, and that includes reading the manuals.