I'm wary of making this too easy. LJ's newish Share button actually prepares your new post with the content of the old post in it - making it pretty much a one-click method of plagiarism.

Of course, if somebody wants to plagiarise they only have to copy/paste... and what we're talking about here is rather different, since it automatically gives a link to the original.

I guess my feeling is that the ability to quote would be nifty, but make sure that it is a deliberate action, and not something that is done by default.