"I wish you were punctual." - "I wish you were a punctual person; I wish you were always punctual; I wish punctuality was part of your personality."
"I wish you would be punctual." - This tends to be used for specific circumstances. "I wish you'd be punctual when we have a date ... when we have an important meeting, etc."
This applies to many adjectives: the first tends to apply to a personality trait, the second to a specific behaviour.
"I wish you were more forgiving (in general).
"I wish you'd be more forgiving (when I use the language wrongly).
Student or Learner