At the moment, if I comment to a message that's a week or two old in someone's LJ, hardly anyone will see it.
The LJ's owner will get a notification by email, if they have that option turned on. Anyone else will have to choose to look at the entry again.
And who, apart from people coming back from holiday, goes back a week, never mind a month, in their friends list? Hardly anyone.
The result is that potentially long-lived discussions are snuffed out in a day or so.
And what happens if someone posts a question for which you'd like to know the answer too? If someone finds the answer a week or so later, and posts it, you'll never see it. Sure, you can add entries to the memories, but how often are you going to look back to see if anything's been added?
cixen and people who read usenet newsgroups will be familar with the idea of offline readers (OLRs). A program collects all the postings made since you last collected them, and sticks them in a local database. If it notices that a discussion thread is continuing - on LJ, comments continue to be made to an old post - it stores them in such a way that it's easy to see that's happening.
Good OLRs let you say 'this thread is important / I want to ignore this thread.' They can point out messages containing key words. They'll let you search postings quickly.
The other advantage is that you can leave reading some threads until later. Sometimes, you'll never read them. But unless you've told the OLR to get rid of them, they'll still be on your machine, ready to be found if wanted.
This is particularly useful with some subjects you're occasionally interested in reading. Say it's cats. Normally, you don't care to read pages of wibble from LJ cat owners saying how much they love their pussy. So you ignore it.
But now your cat has a problem. Has anyone had - and solved - this problem before? Currently, there's no easy way to tell. LJ's search facilities won't tell you. You'll have to look through the cats community (and the kitties one and the pussies one and...) entry by entry.
With an OLR, it's easy. Just tell it to search your local database for the key words... bingo!
Can someone who's seen the source say if it would be difficult to build in the facility to say 'give me all the posts and comments made in this filter group since time t'?