What I commonly see?
Weak punches and blocks.
Holding the breath all the way through the kata.
Robot moves. Lack of natural movement.
Sloppy moves caused by excess speed.
Moves run together to try to keep up with the group in group kata training.
All of these can be fixed in time. Every student is different. Some children simply don't have the fine motor skills to do what is required at first, so we work with what they can do. Many adults have very stilted moves, very stiff and mechanical, and seem to want to develop power by using arm punches rather than the hips. They're often stiff as a board and it can take a really long time for them to relax into the kata and just let it flow.
Adults also tend not to want to bend their knees. Our style is a rather high natural stance compared to most; we don't do super deep stances, yet we still bend the knees and that seems to be difficult for some.