Piazza is a critical part of our teaching experience, and we've heavily redesigned our class to take full advantage of it.
I think the overall Piazza interface--features and simplicity-- is what attracted me in the first place. Beyond that it was just my gut feeling that this might be something useful and my deep desire to interact with the student throughout the course and on a almost constant basis. It has been simply fantastic. It made my interaction with the students very easy and straightforward. BTW...a straw poll of the students in my class indicates that students love Piazza!
I love using Piazza for answering student questions; the user interface is unbelievably nicer than anything else I've seen. It is easy to enter text and easy to see what questions need answering; It is just easy to use.
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 Blackboard, users are automatically signed in to Piazza, so there's no need to remember a separate user name and password.