Apart from stories.
The copy and paste of previous post ends up a complete mess that is impossible to read, as well as boring.
Can the forums be altered?
I always try to edit my quoted chunks so I'm only quoting the pertinent bit I'm referring to before I add my tuppenceworth.
Most people are just lazy and inarticulate/borderline illiterate in some cases though, and a character limit would probably just encourage even more txtspk gobbledygook.