There is often a strong desire to sub-categorize online communities, but the result often ends up just fragmenting the group and diluting anything interesting.

I work in games, and as a result of that (and as a result of it being a hobby of mine as well) end up reading a lot of gaming forums.

The gaming forums which have split themselves organizationally into sub-forums like "First Person Discussion" "RTS Discussion" "Mod-maker and homebrew discussion" "retro discussion!" etc, etc, tend to lack in-depth discussion and long-term vitality, and instead focus almost entirely on surface level discussion.

The most interesting gaming forums, the ones which end up being the most full of life, and have a sense of solidity and community and "place," are the ones which just have just one forum, usually called something like "Gaming Discussion." Within that forum, since it's the only place there to talk, unofficial (but well-regarded) rules tend to form and the community tends to self-regulate.

In places where the community comes pre-segmented by the administration (eg into sub-forums or sub-sections of a site) the assumption seems to be that if you're posting content which vaguely fits within one of the pre-defined sub-sections, you're in the clear regardless of how useless your new thread is. This results in 8 billion threads with titles like "anyone played [game x]?" (often with a single sentence post within, saying something like "i havent but my friend says its lame. thoughts?"), or "fans of [genre x] team up and post here!" (even though that is implied, since you're making this post in the [genre x] sub-forum).

In the less stratified, single "Gaming Discussion" forum layouts, however, the community itself is responsible for segmenting and organizing the conversation in the only way they can -- through the actual threads they create. This significantly raises the implied quality requirements for any new thread (as anything deemed "throwaway" or "quick" like "who is a fan of this?" should either be left unsaid, or should be relegated to a single post inside a larger conversation) and tends to result in a forum filled with fewer new threads, each containing deeper, more thought-out discussion, versus the heaps and heaps of shallow threads that you get in a more stratified gaming forum community.

When there is only one place to post, and the responsibility for segmenting the content therefore falls on the community (by creating new threads), the threshold to propose a new sub-discussion area (a new thread) gets raised far higher, and as a result people post longer within pre-existing threads, causing the discussion to actually last long enough to get beyond the surface level comments which plague more pre-stratified and pre-moderated discussions, and that style of conversation eventually seeps back up to the top and carries into the beginnings of the next new thread.

Look at the forums on the gaming meganews site gamespot.com for instance, which is stratified down not just to different genres, but to a sub-forum for each individual game (and is considered a wasteland, consisting of nearly entirely "when is this game coming out?" and "I liked this lol" threads), versus the single-forum communities at www.neogaf.com and the Something Awful gaming forum (which are both stable enough to be populated with fans and developers and press alike all on the same board, and are both frequently visited as sources of news and interesting discussion by all of those groups).

Of course, these are gaming forums I'm talking about, so there is always going to be significantly more idiocy than something like Hacker News. So, to the untrained eye I'd imagine even NeoGAF or the Something Awful forums may look like a wasteland devoid of content, but (with a little investigation) relative to the rest of the gaming community/discussion landscape, their content and vitality speak for themselves.