Well, I've been in on my share of Internet discussions, and the most common debates center around matters of opinion, or unprovable facts.
We have the "is napster breaking laws" debate, quite honestly, the answer is... none of us techy types really know. I don't know copyright law, and I don't know, for sure that they ARE breaking laws, yet for months, the debate raged on, unabated.
Of course there's the "Linux Vs Windows" HOLY WAR, and that really (let's face it, everyone), comes down to a matter of opinion, in all honestly, I can't say either OS is truly "better" than the other, I just happen to use windows more, because the work I do is in windows, and therefore it is the most useful environment at home (work at home, etc), not that I don't like messing around with linux from time to tome.
On the gaming forums, we have the 3dfx vs. Nvidia debates, which, for a while boiled down to speed statistics and company loyalty (fanboys), but has now truly become another "image quality vs. speed" debate, which, settles back down into that opinion niche.
Even the KDE vs. Gnome battle comes down to opinion, even if it's just the opinion that Open Source is inherently better than closed source (and even now that's not really the issue at hand).
The truth is, I don't think opinion debates are very useful at all, because it's impossible for anyone to defend themselves. With the web as big as it is, most non-opinion debates boil down into someone finding a link to the facts and posting it, thus ending the debate.
Not that opinion debates don't have their place, they do, I think people should get a chance to see a wide variety of viewpoints, I just don't think they need to be carried as far as they generally are.
Oddly, debating about opinion seems to have nothing to do with sex, intelligence, age, or experience. Even as I decry the useless opinion debates around me, I often find myself getting into them. Perhaps it's just human nature.