I wasn't going to post on this, but I've noticed one item missing from this discussion, and it's the magical ability of diamond mods only: That of locking a post.
Certainly, for our historical posts which we don't necessarily want to delete, but also don't necessarily want to encourage copycats in the future (EG, the polls), a lock with an edit by the diamond mod who's locking it might go a long way. For example, placing a disclaimer similar to the following at the top of the post:
This post is from the early days of [Stack Overflow|Server Fault|Super User], and while we recognize its historical significance, we also recognize that the current community of users will likely close similar posts. Please feel free to read and learn from this post, but refrain from creating similar posts just because this one exists.
Just a thought. It gets us past the "deletion" problem (IE, continuing to provide to Google Juice and historic reference we all love), while also discouraging future users from citing this as a reason their off-topic and typically closed question should remain.
As a further bonus -- this one just occurred to me -- locking makes it really difficult for the post to "bump" -- only Community will be able to do that, as no new answers, no new comments, not even edits are available.