"I'm done" threads actually have a purpose though. Some people have limited time and limited money. They don't have the time/resources to invest in several game products, and have chosen D&D as the game to go with. In my case, I've probably got over $1000 invested in D&D books and thousands of hours of my personal time. Recently I came to the realization that D&D isn't the system for me, and I made the switch. In my case, it was to GURPS, but that isn't the point.
The point is that when you have so much time and money wrapped up into something, quitting it cold turkey can be distressing. It may sound silly to some, but I imagine "I'm done." threads serve the same purpose as "We broke up" threads. People just want to talk about it to help them deal with it.
Couple that with the fact that ENWorld is the most friendly online community I've ever encountered, and you can see why people would come here to talk about their recent "break up." They are looking to talk to people who have had similar experiences and seek justification for their choice.
I don't think it's about trolling and trying to stir up a hot debate, even if that's what it usually degenerates into.
That said, it'd be nice to have a non-d20 forum. I realize ENWorld is devoted to d20, but many members of the community play other games and wish to discuss them with this community instead of going to another site.