You are no longer wondering what will keep the students engaged in class-you KNOW. At least, you know the reverse-what will NOT keep them engaged in class..
You stop telling them tales of how great your school, college days were..because (see above).
You start mentally grading a question that someone has asked, as soon as he has asked it.
You have five (or more) final exams potentially ready, and you just have to decide which one to use.
You know what used to be an easy question once may be a tough question today. :)