Well, blogging is dead now, so maybe it makes sense for Twitter to turn into blogging.
I'm really interested in how they go about this without destroying the essential Twitterness of Twitter. Google+ does something like this: posts are auto-folded after a few hundred characters. It does encourage you to get to the point--obviously nobody's going to click "Read more" if there's nothing interesting above the fold--but it certainly does not lead to posts that are typically less than 140 characters.
Facebook used to have a limit of about 450 characters or so before they increased it. Most posts there still aren't anywhere near 10,000 characters, but I think a lot are probably over 450.
Facebook used to make Heebie refer to herself in the third person.
Bob Dole thinks that is fine.
This seemed right: "What's really changing here, then, is not the length of the tweet. It's where that link at the bottom takes you when you click on it--or, rather, where it doesn't take you. Instead of funneling traffic to blogs, news sites, and other sites around the Web, the "read more" button will keep you playing in Twitter's own garden."
7 and its link seem right to me. This is all about wall-building.
I'd applaud if they limited it to zero.