I think the main reason I find TGGD so frustrating is that everybody involved usually seems to be incredibly strongly convinced of their own rightness and the wrongness of all other positions, but nobody can actually prove anything on either side. What results is at best a tedious stating and restating of positions, at worst a furious battle with both sides throwing axioms at each other. I don't know if there's any recorded evidence of someone changing their mind about religion as a result of a usenet debate, but I've certainly never seen it. There's an obvious reason for this: you can't reason someone out of a position they didn't reason themself into. You're using the wrong tools: not so much using a sledgehammer to crack a nut, more like using a fish to change a bicycle tyre. The only possible exception to this, as I see it, is that opinions are sometimes changed by ad hominem arguments: if someone repeatedly behaves cretinously in TGGD, their whole belief structure may be (however unfairly) forever tainted in your mind as a result. But essentially, the debate is unscientific; I don't mean that in a pejorative sense, I just mean that trying to subject it to scientific analysis is not necessarily a useful or interesting approach.
Are there interesting conversations to be had about religion? Of course. And that's part of the frustration: all that potential interest being reduced to a mud-slinging match between a bunch of people who seem to think (in the face of libraryfuls of evidence to the contrary) that the only reason the question of Whether God Exists And If He Does What About Dinosaurs And Cancer Then hasn't been answered yet is that not enough has been written about it yet. Unfortunately most of the interesting conversations would involve starting from the assumption that belief systems other than one's own may contain something of interest, and this often doesn't seem to happen. There are plenty of interesting conversations to be had about the benefits of religion to society and the individual, about the separation (or otherwise) of church and state, about the history of religion... No, please don't leap in and say "other people's delusions/fairytales are fundamentally boring"; they may be of no interest to you, but stories can be interesting, the shared narratives around which people weave society can be interesting, delusions can be psychologically and philosophically interesting. Really, trust me on this: lots of things which are not maths can be interesting. However, everybody in TGGD is generally so entrenched in their positions vis à vis the wrongness and stupidity of everybody else that the necessary mutual respect tends to be lacking.
I think there is something inherent to debates concerning religion which, while it doesn't inevitably cause this sort of attitude, certainly helps it take hold: that is, the ever-present implication of Judgement. It is difficult (though not impossible) to exchange ideas on an equal footing with someone whom (you assume) believes that you are damned to eternal torment, or indeed (from the other side) with someone whom you regard as having condemned themselves to a fate worse than death. There is an unequal relationship there, where both sides think the other side is sadly misguided but could be saved from their misguidedness if they would only listen. Of course, by "listen" both sides mean "change sides": because all sides believe that they are self-evidently right, the idea that someone could listen to the message, hear it and still disagree with it is, well, beyond belief. So they shout louder. AND LOUDER.
The real tiresomeness of the debate, though, is that attempts to say "this is tiresome, can we have a more interesting conversation?" will always be interpreted as a desperate attempt to have the last word. Once you're in, there's no way out. Starting TGGD is the conversational equivalent of declaring thermonuclear war (but with fewer actual explosions and more sarcasm) and we all know what the only winning move is.
 I have a marvellous proof of the (non-)existence of God which this post is too short to contain. NOT REALLY.
 This isn't original, but I don't know who said it. Ralph Waldo Emerson, probably.
 At this point I was going to digress briefly into the question of what constitutes an "interesting" conversation, but I don't have time if I'm going to get something posted today.
 Okay, okay, everything's maths really. It's numbers all the way down. But you know what I mean.
 I admit, this risked turning into a massive rant about the standard oxbridge.tat debate, which basically boils down to drawing a raggedy line IN BLOOD between Oxford/Humanities and Cambridge/Sciences, but that just made me lose the will to live.
 As I am somewhat vis à vis the oxbridge.jihad above. I made no claim to be impartial.