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Threaded comments are a great addition; many blogs, including Mashable, have switched from a single-threaded to a multi-threaded commenting system and never looked back. The ability to see some extra info about the user that left a comment, including his comment count, is obviously quite useful, too.
But the really big news is the voting system, which gives users the ability to rate a comment positively or negatively, similar to Digg's voting system. The votes result in a comment rating, which can be really useful if Facebook plans to introduce a comment moderation system based on the ratings later on.
We've asked for Facebook's comment on the screenshot above, but haven't yet heard from the company.