Some examples:
1...not listening out (not alone here,but the worst offenders)
2...Either unable to follow route instructions, or read them back in less than 3 attempts
3...asking silly questions on extremely busy frequencies like "can we direct...." despite just having been given a radar heading.
4...referring to flight levels as if they were altitudes.
I could go on but then I`d start to sound like one of them.