Only questions with more than 1/3 of students making mistakes are discussed.
Students received a report card from the testing center. The report card include a total score for the exam, the questions missed and student's own answer. Students are encouraged to come to office hours to discuss each question they missed.
Scores are usually summarized using stem-leave plot along with mean, median, highest, lowest, standard deviation.
They are not given access to an answer key. However, each student is encouraged to come to discuss their work during office hours.
The exam is scored right after finishing the exam. They immediately know there scores and questions they missed. The comments made in class are usually about the performance of the whole class, and discussion of questions missing by more than 1/3 of students. Students usually use office hours to talk about individual performance.
The system I use perhaps more close to losing points for mistakes.