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If the comments are hosted on the commenter’s server rather than the poster’s server, then you’ll need some middleware caching layer that can pull in hundreds or thousands of bits and pieces of a thread from servers all over the world in different network conditions and build out the thread fast, in addition to dealing with missing or altered comments. (But on the plus side, you could get away with less on the moderation and legal side).
All of which needs to be as simple to set up and maintain as clicking on an icon and typing in a username and password. Oh, and needs to be free (gratis), of course.
