Jorge,
I have never understood forum designer's aversions to subforums. The very best forums i have ever used had not just 1 level of subforum, but two. ITems were always bubbled up to the current level, or you could drill down for browsing. This was how usenet, source, and TheWell forums akways worked, back before OpenPop, vBulletin and Phpbb institutionalized the flat/linear, convervation style board boards that are popular with kids, and hated by most of us graybeards.

While linear boards are nice for discussing random topics, or what you have for breakfast, they are nearly useless for comprehensive technical discussion, or anything that requires organization of ideas or action items. You cannot rely on users to create topic threads. But a simple, 3 level hierarchy makes MOST organization, even fairly complete ones, easy. Whether you allow child messages to appear at each level is optional.


Liferay
> General Q&A
>> Topic1
> Portlets
>> Forum
>> Wiki
>> Private Messages
>> Calendar
> Portal

Liferay Documentation
> Liferay FAQ
>> Roadmap
>> Version1
> Liferay Quickstart
>> Roadmap
>> Version1
> LiferayPedia
>> Roadmap
>>> Topic1
>> Version1

Everyone always tries to break away from hierarchies, it always fails, and people scream for more organization and more structure. Then a Plone comes along and peopel say "Wow! Hierarchies! Cool idea!" Of course, in between, you get tagging, which was the last "big idea" that programmers felt could replace precoordindate indexing. Surprise! It can't. Humans like order. Tags are far too random for anything but very small, or very large collections (where there's little choice). I have never seen them replace a good structure (notice how Google was pressured into adding label groups, and coming soon, sub-groups).

A three level hierarchy will change everything about these forums. Naturally, the 3rd level is optional from forum to forum. And no, having 3 does NOT mean you must keep on going down to a 4th, 5th, etc. If 3 levels is generally good enough for Dewey decimal structure, it's adequate for discussion forums. I've worked around libary systems for 25 years. It is nearly impossible to branch anything concisely in only two levels. If you could, a lot of librarians would be out of work. Instead, they are the cool new occupation title (according to a recent New York Times article).