Have a look at this thread:
https://www.newschoolers.com/forum/thread/768122/The-Official---What-is-wrong-with-my-boots---Thread?
But off the top of my head the main issues people have with their boots are:
1. Not bothering to make an effort to drive to a good boot fitter and just buying boots online. Boots aren't like skis, jackets, bindings, etc you can't just look at the specs and make a purchase based on measurements and product descriptions. You actually have to go and try the boots on and get them working for your foot.
2. Not investing in custom footbeds. An excellent analogy made by onenerdykid was that your stock powerful 100+ flex boot without a custom footbed is like a high performance car, however it has the steering precision of tractor. The footbed in a boot acts as the high performance steering mechanism which gives you the control you need. Without the footbed your squishy foot is just going to be mashed around in the rigid ski boot.
3. Buying boots too big. I made this mistake when I first started, bought boots to big because they felt like nice warm roomy slippers in the shop. All I can say is this lead to the complete destruction of my shins and feet. When you buy boots in a shop they should be very tight, not so tight your feet go numb and starting turning purple but tight enough that it feels as if your feet are held in place and not moving about. If you do a shell check there should be 2cm or less space between your heel and the back of the shell. After a day or two of skiing the liner of the boot packs out and give you an extra half to whole size.