Here are a few WIBNIs, all for comment notifications:
1. Could the journal/community name that a comment is posted in be included in at the top of the email notification? Or even in the title (“Reply to a comment in lj-dev...” for example). This would make scanning, and potentially sorting, of notifications much easier.
2. Being able to unsubscribe to subthreads would be useful, to cut out notifications for occasional endless private wibblefests. The currently pointless pushpin in a monitored thread could be reallocated as “don’t notify anything that gets commented to this” - I’ll grant that this presents a slight extra challenge to coding the post-posting queries efficiently, but (without seeing the current implementation) I can see it as entirely feasible.
3. If I subscribe to a posting or a thread and somebody comments to a comment I posted, I get two notifications for the same message. This does no harm, but an option to stop it would save me a small irritation.
4. Contrariwise, comments in watched threads that I post aren’t notified to me. No reason they should be, I hear you say, but if I (or, more likely, someone else with more tuits) were to write an OLR for thread monitoring - so people could see their emailed notifications in thread context with “read” flag processing - getting my own comments would be useful.
5. Further to 4., a third XML part to the multipart/alternative would be handy to the same end, aiding machine parsing.