What I like about Piazza is that you took something that used to be a mainstay at Ga Tech (Usenix Newsgroups) and brought them into the 21st century on the web and you did it right.
Piazza is an amazing way to keep tabs on the pulse of your class. Student posts give me timely feedback on what concepts need clarification, provide live feedback during in-class group activities, communicate issues about how we run the course and why, and allow students to share science finds in articles or videos. I've found some great teaching material in articles and videos. The additional bonus is how much lighter my e-mail inbox is for BIOL 1510.
Piazza is very nice, my students are enjoying it. I particularly like being able to use LaTex commands to display the mathematical formulas. We hold on-line review sessions before every test, and it's much neater than the whiteboard options.
Instructors can create a Piazza for their class with a single click.
Students and instructors are automatically enrolled with appropriate roles, based on the course roster.
When clicking on Piazza in T-Square, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.