"Receive E-mail for friend's comments"
Short, concise description of the idea
An option that allows you to receive an e-mail when your *friends* posts get comments.
Full description of the idea
I've got a lot of people on my friends list - not as many as some - and I've noticed that I end up not noticing conversations because they're not on the first page of friends, or I just plain don't notice. I already get emails when someone posts a comment on my journal - how about if LJ would send emails when someone posts a comment on any of your friend's journals? Or perhaps just selected friends.
An ordered list of benefits
Makes it easier to track conversations on friends journals, obviously :P Less obviously, decreases re-reading comments because you can't remember if you've seen that already - minus CPU time, minus bandwidth.
An ordered list of problems/issues involved
CPU time and bandwidth :P Each comment would have to be tested to see if it needs to be dispatched via e-mail. This might be a quick process - I don't know the database system well enough to give any good suggestions on how to implement it though. You'd also have to be a bit careful to censor replies to friends-only or friends-groups posts.
An organized list, or a few short paragraphs detailing suggestions for implementation
Perhaps a maintained table on "people-to-send-comment-emails-to", organized by the ID of the person whose journal is being commented in. Except I don't know any efficient way to manage variable-sized lists of numbers, but then, I don't know databases at all well :/ I don't know how fast or how slow this would be, but note that it doesn't need to be an instant-reply thing - a delay of a few minutes would be unnoticed.
Perhaps make this a paid-users feature, and set up a dedicated box with that particular table? I don't know if it would be faster to check comments as they're posted, or scan the latest comments every five minutes or so to make it a larger batch access.
Am I explaining this better than I think I am? >_<
I'll (try to) code it if people want - it's just a matter of, um, learning another programming language :/