How about deleting threads that are duplicates.
Not the same subject (although sometimes with new members come the same topics), but when a person posts the same thread in different forums.
I don't agree with this ~_o
Many people will post stuff in 2 or 3 different forums, just to get more opinions. Different groups of people have different opinions, so asking these different groups for a more "overall" opinion is good. Just because you happen to see it two or three times doesn't mean EVERYBODY will see it many times.
I say keep them in the different forums, as long as it's relevant to the forum. If it's somehow off-topic in the forum it's in, then it should be deleted if it's found in a better location already.