Speaking from the repair side, the problem I've found on the majority of automotive forums is that there are people posting misinformation. While there are always a few really good techs posting on most boards, some of the replies are a waste of time and money, and some can damage your vehicle.
But more than that, it's hard to break down some of the more complex problems into a user freindly diagnosis/test/repair for the home wrencher.
It's not that they wouldn't understand the answer, but when posting a possible cause or test to help someone who does his own repairs, I have to consider the likeliness that he may not have the same diagnostic equipment that I use. This makes posting more difficult because many systems today require special tools or procedures to troubleshoot.
What I do in these cases -where possible- is post alternative diagnostic methods which can be used in lieu of expensive tools. Coming up in this field I didn't have the money to buy a $5K scantool so I learned to work around this for a while and I share that info when it's helpful. I figure that it probably wouldn't be worth it for the home mechanic to buy these tools unless they have enough bank, and really enjoy doing thier own work.
Even the highly rated IATN network of proffesional techs isn't that good, well you can get solid info there on just about any vehicle/problem, but you have to know which responses would be condusive to getting the job done, and which are not, but many of them require use of advanced diagnostic tools which most shade tree techs don't own.
I used to post on Pro-Autotech.com, and while some of the good techs moved on, they still have a few pros that give solid advice. As with most auto sites you just have to sort through it and use what is relevant to the problem. I post on a few boards when I can, but I can't say any of them would be the ARFCOM of autotech boards.
So in a nutshell, albeit a large nutshell, this is why I find it hard to endorse a car repair forum as compared to ARFCOM, those are big shoes to fill.