Having used to be mod/admin on a rather busy and strife-filled SMF-forum the past 6-7 years, I can confirm that this works surprisingly well.
I say "surprisingly" because, like a lot of tech hackers, I often think about a technical solution before I think about the simple social one. But I've noticed time and time again just how much effect the simplest changes in user-interface can have on the entire "mood" of a community. And "simple change in UI", includes the guidelines, or--I don't know--there's actually a lot of space for a notice below the comment submission form where you can give people the "right" idea (what you want to see in this community).
The personal anecdote goes a little something like this: We have a real solid group of mod/admins with a hands-off approach (way more hands-off than the invisible ones at HN, but that's a different gripe), so we had our fun with the innocent tiny changes, like changing the descriptions of individual sub-forums ever-so-slightly, like a pun, or a little in-joke. After a few years however, all of the descriptions had mutated into something that was not at all descriptive anymore. All the "old" members (which includes the mods of course) knew perfectly well what the sub-forums were meant for. But the new members did not. As new members look to older members for social context, there was a sort of generational loss in relevance, and at some point people almost entirely just posted whatever, wherever. It was chaos! And so I had the brilliant idea that, hey we can change those descriptions so that they're both descriptive and somewhat funny (utter genius, I know).
The amazing thing is, that it took only about 2-3 weeks before people starting posting on-topic things in the right sub-forums again.
The moral of the story is: people really tend to respond really well to just flat-out being told what to do. Really well. Amazingly well. Frighteningly well.
Better than some tech solution that tries to subtly "guide" them, in particular (or at least much easier to implement).