Too slow at altitude
I fly corporate jets certified to 49000 and 51000 respectively. The number one issue regarding high altitude flight is speed. Do not let it get loe, climb at 100 feet per minute if you have to. Or go back down again. I used to fly with a guy who would follow the profile all the way to altitude, and we would not accelerate at all after level off. Going that slow makes it feel like you are balancing on a beech ball. A little turbulence can be enough to make you soil your shorts.
These are things that you should know by the time you are the PIC on a jet.