It's not exactly vital for SEO, but more for the user of the search engine results as they see a description and may decide to click or not. Bbut as Jeremy said it is from the first post.
You could edit the initial text of the first post to suit, but the poster may not be too impressed.
I think in the end it's best to let this work itself out organically and not worry about SEO at thread level. Xenforo is basically quite good for SEO , but as with any site, content is what counts and with a forum you have less control over content as opposed to other sites or blogs.
Weeding out thin content (threads with no replies etc.) may help, but in the end I feel life is too short to worry. A good forum with a thriving community that discusses stuff intelligently based on your niche will win out, especially if it's on xenforo as opposed to that other forum software.