I also think removing a decade or more of social threads alone, plus other zombie threads, would help. Someday I'll get the courage to submit something to SYW and I like offering feedback when I can. It's big for me. The Research forum has been invaluable, and I'm learning a lot from the Outwitting Writer's Block section. Someone else pointed out that some of the older posts might be removed from SYW, too; I think that's an option as well. Limit little-used emoticons...I agree largely with quite a few ideas that have been suggested. However, I will note that my ideas for the site are less "trim the fat" and more realistically viewed as "cut off entire hunks." I would like the site to stay up and I really want to stay.
I hope that helps!