I've noticed multiple problems when I pay attention to under performing players.

1. They press buttons as if they have a 5-10s gcd.
2. They don't know what weaving is.(saw a dragoon use power surge+jump+spineshatter+dragonfiredive back to back without touching a single gcd.
3. Don't even know what their abillities do and how they affect one another(ice mages)
4. Fail to use Dots and maintain them.(probably assume they are not worth using)
5. Unnecessary movements.(positionals and hazard zone avoidance)
6. Failure to move at all.(Positionals and hazard zone avoidance)
7. Abillity to make good judgment.(Using touch of death on a mob with 5% hp that is being focused down.)
8. Netflix.(I wish I was joking)