Here's how I view it:
In a discussion, the intent isn't to persuade: it's to collaboratively build. The basic assumption is that everyone's viewpoint is basically valid and interesting and deserves expansion.
In a debate, the intent is to persuade the audience, through rhetoric, argumentation, or what-have-you, to a specific point of view. Usually, multiple points of view are presented by different authors, all of whom are trying either to prove their own point, disprove the other points, or both.
Maybe examples are better.
Discussion:
bobtheskull: I think X.
lightsabre: I think Y.
bobtheskull: That's interesting: that completely changes the nature of Q, though!
lightsabre: Hnh, you're right. But if Q is that way, then that means that X and Y are both really close together.
someone else: I have this theory about N: how does that fit into X, Y and Q?
bobtheskull: Weeeeeell, Y supports it, but not X, unless you also take Q into account.
lightsabre: Hold on, X also supports it even without Q, because check out this neat thing!
bobtheskull: Holy cow, I never even saw that. Did you know about R?
someone else: Wait, Q and R have to do with X? HOLY CRAP, JIM'S A GENIUS.
bobtheskull: Amen.
lightsabre: Amen.
everyone else: Amen. Hummus?
bobtheskull: Dude, awesome hummus.
lightsabre: TOTALLY awesome hummus.
someone else: Wow, what's hummus?
bobtheskull: Chickpeas.
lightsabre: Chickpeas AND olive oil.
someone else: Chickpeas, olive oil, and Angelina Jolie.
everyone else: *drool*
Debate:
bobtheskull: I think X.
lightsabre: I think Y.
bobtheskull: X is clearly right, because of A, B, C and N.
lightsabre: Y is clearly right, because of Q, R, S and T. Besides which, X is invalid on its face: C isn't canon!
bobtheskull: Anyone who's read the books knows that C is canon. X is totally valid, because of A, B and N in addition to C. Furthermore, Y is wrong because of...
someone else: I totally disagree with both of you. P is the only right answer: X is invalid because of C, Y is invalid because of S, and P incorporates the best parts of A, B, N, Q, R /and/ T!
bobtheskull: Okay, point, but, dude, B.
lightsabre: Forget B, check out Q! I still believe fervently that Y is the way to go.
bobtheskull: I disagree, but I see your point.
someone else: I disagree with bobtheskull, and kind of agree with lightsabre, but I still think my P is the best idea.
someone else: Heh, P. Geddit? Pee? HAHA. Er. Anyway, she has a point. All in favor of P?
someone else: I like P, too. I can't believe I wrote that.
someone else: No way, bob's totally sold me on X.
someone else: Eenh, I'm with lightsabre on Y.
someone else: Man, I'm hungry. I could totally go for some hummus.
bobtheskull: I'm in favor of hummus. Well argued, lightsabre.
lightsabre: I third hummus. Back at you, bob.
someone else: Awesome, hummus! Come on, GROUP HUG!
everyone else: *hug* *hummus*
Just Plain Dumb:
bobtheskull: I think X.
lightsabre: I think Y.
bobtheskull: You're an idiot.
lightsabre: Am not. YOU'RE an idiot.
bobtheskull: You're /always/ an idiot! Look at (unrelated thread)! You always do X!
lightsabre: I am not. I NEVER do X! Except that one time, but that was because YOU did X, I was just defending myself!
bobtheskull:OH YEAH, WELL I CAN TALK LOUDER.
lightsabre: YEAH, WELL, I CAN BE MORE VICTIMIZED.
someone else: You know, maybe this argument isn't a good idea.
bobtheskull: SHUT UP I'M REAMING THE IDIOT.
lightsabre: YEAH WELL I'M REAMING UR MOM.
someone else: JESUS, SHUT UP.
bobtheskull: NO, YOU SHUT UP.
lightsabre: NO, YOU.
everyone else: (stabbing selves in eye with fork)
iago, mickeyfinn, ashton: *BANHAMMER*