I'm relatively new to this forum and I hear what you're say'n but all the negatives come with the territory. It's the nature of the medium unfortunately ...
You also have to remember that its not costing you anything to join most forums, unless that forum offers say a paid subscription level membership that possibly includes other services: e.g buy and sell, etc.
I belong to several photography forums: film and digital; 35mm and larger formats; astronomy, etc., and without fail, there will be a few individuals that like to stir the pot; spew vitriol; act up and act out, etc. etc. etc. As long as Internet forums in general allow you to be anonymous in some form or fashion, you're gonna have individuals who can't be held accountable for the things they post.
What separates some forums from others is how engaged the moderators are at monitoring their respective site and whether or not the rules are enforced. Some forums give what I'll refer to as a "cooling off period," if you're a repeat offender. In other words, they'll block your account from posting any comments or starting a thread for say 24 - 36 hrs., so that hopefully, you see the light and mend your ways.
Other forums have tried deleting a users account after egregious violations. Unfortunately, that doesn't really work because a person can always create another account with a different alias. But after awhile, you recognize them for who they are. As for the repetition of topics and questions - again, its the nature of the beast. Most search engine features (if you bother to use a search feature) on a site or anywhere else are gonna return way too many search results that are gonna be difficult to narrow down to what you're looking for - and sometimes, you just don't know. It's hit or miss.
Can't find what your looking for? = Start a new thread with (you guessed it) ...
Maybe tailor some FAQ sticky's that can address most of the general questions people are going to ask.
But that becomes a slippery slope. Where do you draw the line?
Bottomline: You gotta take the bitter with the sweet. I know its a cliche but its true. Learn what you can and ignore the rest. Don't engage in tit-for-tat; don't insult total strangers; don't fall for the rope-a-dope, or reel-you-in tirades that push all your buttons and suck you into a nasty verbal exchange.
You're not required to play that game.
If you can answer someone's inquiry, lend a bit of constructive assistance or guidance, by all means.
We learn from each other - but being rude and bellicose in the process is a choice. It doesn't have to be yours.