I'm just gonna quote
the post here, because...yeah.
Comment subject lines are back!
We're pleased to let you know comment subject lines are returning to the site's default commenting style! Comment subject lines were only available when viewing custom comment pages, but they are now included as an optional setting for standard comment pages as well. You can turn comment subject lines on for your journal or community by visiting
http://www.livejournal.com/customize/options.bml and enabling the "Show subject in comment form for system style of comments" setting (the wording of this setting is likely to change soon).
Edited to add: The new option will only display if you're actually using system comment pages. So if you're using custom comment pages (if you have the "Disable customized comment pages for your journal" field set to No), then you won't see this option. If you want to use the system comment pages, you need to first set the "Disable customized comment pages for your journal" to Yes and save the page, and then you'll see the new option appear right below that one. You can then use the new option to turn on comment subject lines.
It's not perfect (in order to make the subject line a link, they lost the black text so it blends a bit with the username and other linked data in the comment header bar) but it's still a clear sign of the upcoming zombie apocalypse.