Maybe some sort of rating system is the answer. Unless I'm mistaken, Slashdot does something like this, but on an individual post level. Maybe when you read a thread you could rate it on some sort of scale (1-5 for example) according to how helpful it is. The scores of all the people reading the thread could be averaged, giving each thread an overall score. Then if everyone participated in rating the threads the useful threads would have a higher score and the less useful threads ("how do I hook this up to my car" for example) would have a lower score. The oldsters who have been around forever and know what's been covered in the past a little better could even have their ratings weighted a little higher.
Ideally, this could keep the informative threads accessible by filtering out the junk. When we search on a topic we could decide how deeply we'd like to search. Of course you could still choose to search all threads, but you'd also have the option to search for "3 and above", "4 and above", etc.
That's my 1-3/4 cents. I have no idea how it would be implemented or whether people would participate, but it's a thought.