As I see it, the difference between "discussion" and "debate" is the same as the difference between "black" magic" and "white"...ie, it lies in the realm of "motive and intent."
In a discussion, your motive is to share your thoughts, insights, and questions with a like minded audience, and your intent is to learn and gain new insights from other people's perspectives. As online communications go, the process is collaborative.
In a debate, however, your motive is to present your thoughts, insights, and questions to the same audience, but your intent is to convince them that your perspective is the correct one. As online communications go, the process is competitive.
Personally, I love a good academic debate...and I know from experience they can be done effectively on a forum such as this one...as long as the "Rules of Engagement" are called out ahead of time; and all parties agree to abide by them.
Rather like a Wizard's Duel, wot?
Edited: 'Cause I can't spell...