According to Donncha O Caoimh a new version of WP Super Cache is available.
This release addresses some issues large or busy sites had with deleting expired cache files. Some sites generated so many cache files that there were two [sic] many files generated, even when those files were cleared out every hour. To fix that I limited the recursive function that deletes the files to 100 deletions at a time. That function is called until all the cache files are deleted (or strictly speaking, no files are deleted. That’s how …

According to Mark Jaquith:
Subscribe to Comments puts a checkbox in your comments form that, if checked when leaving a comment, will e-mail that commenter all followup comments on that post.
This is great news for those using WordPress.com. It should help increase the number of comments, and increase discussion because of comments. Great Work!