I agree, defaulting the forum to showing "open threads only" seems like a good idea (with the option to see all of the entries by clicking the link in the right column.) I don't delete the expired posts because the search engines cache them and I don't want to lose any new people that come in through that way. Personally, I don't like being automatically re-directed to another page when visiting a new site.
I do the majority of marking deals as expired but I miss some, particularly when posters don't include the expiration date.
There are a few that I need to investigate to see if they are expired or not, but I think that will happen after Christmas.
Merry Christmas!