I prefer forums with high expertise.
The forums on www.backpackinglight.com
have been traditionally pretty geeky, i.e., folks more interested in digging in to details, weighing things, trying to test for warmth or waterproofness or whatever in objective ways --- or at least there was some of that.
At some point they started to require a paid subscription in order to post. I had been grandfathered in there but then they screwed that up, and I refuse to pay to post. And once I couldn't post, I found myself uninterested in following any discussions.
So what I recollect might be way off now, but you might have a look.
Some forum discussions on whiteblaze.net have decent expertise, but I think that in any forum you have to filter through a lot of other posts to find those.
Partly you start to sort of calibrate based on the poster; perhaps one person is really into stoves (in fact, I find that a surprising number of people are really into stoves ... and knifes ... and a few other things ...). So you search for their posts on the topic if it interests you. Another might be a demonstrated expert on a particular type or brand of tent. Etc.
The other thing is to just learn how to effectively search the various forums. Most topics have been talked through to a great degree, and if you can sort and filter through what's already been said on a topic that interests you, that can often turn up something helpful. I suggest that, in addition, or perhaps instead of the search box associated with the forum, try a domain-constrained search using google or whatever is your search engine(s) of choice.
An example of a domain constrained search would be to put this text in the google search box:
site:whiteblaze.net rain jacket hydrostatic
Just searching a forum site for "best rain jacket" is going to of course get you a lot of hits. But if you want the more geeky responses, knowing a that-piece-of-gear-specific geek word and putting it into your search can make a big difference in the type and level of detail of the hits you turn up.