After a certain amount of time, threads are automatically closed to new comments. The length of time varies on different parts of the site.

MetaFilter - 30 days
AskMetafilter - one year
Projects - never
Music - 30 days
Jobs - there are no comments on Jobs
Podcast - 30 days
IRL - 6 months
MetaTalk - 30 days

On MetaTalk and IRL only, a thread can be closed by an admin but not deleted - such threads remain visible on the main MetaTalk or IRL page. (See here for more on why this happens.) Note that this fuctionality does not exist on the other subsites, so for example it is not possible for an admin or a user to "close" an AskMe thread without deleting it.
(tags: threads comments closed ) August 2, 2006 - permalink - back to questions