Are we discussing "lounge" sub-forums ?
If not ...
... as a self confessed ludite ...
If a thread is still relevant,
maybe leave it open
But if the content is outdated, let it go. Lock it or move it to a legacy forum.
To retain an old (unlocked) thread because of an emotional attachment defeats the purpose of any forum.
And if a thread is that brilliant it should be a sticky or tutorial. Maybe rewritten by admin (or knowledgeable volunteers) to cut down on the crud and misinformation.
Locked threads discourage "ranters" (rants only bump threads, and again defeat the purpose of the forum)
I choose a forum by ...
- number of lookers on line.
- age of first page of threads
- how active is the forum
- how quickly are threads responded to, if at all.
- quality of support -- one line answers can just be post-counting.
- stickies or tutorials
- logical structure --some forum structures out smart themselves
- no ads when I am logged in
This is the only reasonably active forum for "eBooks" I could find. The replies have been excellent and very helpful.
Timely, to-the-point, address my questions
There is nothing more irritating than spending an hour or more reading through an ancient thread to discover the solutions/structures have changed over the years.
eg, I certainly don't want to waste my time reading about Win-95 in 2014. 
In their frustration newbies want info/help that is relevant NOW. As dinosaurs we grew up in times when we had to be patient there was no internet nor complex home technology.
On forums where I know my stuff there are a lot of young people and dinosaurs climbing the walls out of frustration on how to resolve their technical problems.
"Balance" is the keyword.
If we live in the past we are ignoring today.
