I think this was a fine post that gave me something to think about, but I disagree.
Generally, people on forums get angry if they DO NOT get any feedback. Then things get ugly because that is frustrating.
If developers are relatively active (they should at least answer when they are directly asked something) then a forum will most likely run nicely and be a constructive place.
Also, people have to feel that problems are being fixed, or they will get annoyed.
Of course, there are always morons (this is the Internet we are talking about, after all) so there is need for moderators to warn/ban in order to ensure that the community is as pleasant as possible.
I have two examples to back up my views.
The Sports Interactive forum (they make the Football Manager series): http://community.sigames.com/
This is an example of a very good forum that is quite constructive. Sure, sometimes there are rants and so on, but it doesn't seem to scare the developers who post A LOT, and certainly answer all (semi-)important questions. This ensures a constructive atmosphere. Of course, the moderators are very active also in order to close ridiculous threads and so on.
An example of an impossibly bad forum, on the other hand, is the EA Sports FIFA forum: http://forum.ea.com/uk/categories/show/10.page
In here, the developers NEVER answer anything, and a few worthless community managers once in a while (rarely) give the most feeble and useless replies.
Also, people are NEVER banned or even warned no matter how amazingly stupid they act. The result is a forum that is a complete mess. A billion threads are created and people are almost constantly angry and abusive (it sure doesn't help either, of course, that almost no problems in FIFA are fixed).
Probably the target audience for FIFA is somewhat less mature than that for FM, but still I think moderation and developer replies
are key to having a constructive community.