Apologies if this has already been suggested and kicked out!
In fast moving threads (on AIBU for eg) cross posting is an inevitability but can get very confusing, because MNers refer to previous opinions which may well be a long way up thread by the time their message appears.
People can (and do!) overcome this by cutting and pasting relevant bits of a previous post and/or citing the date and time of the message but this in itself can become unwieldy. If however each post within a thread had a number (starting with 1 then 2, 3....you can see where I'm going with this 
) then referencing the post you are replying to would become a breeze!
To make things even easier, in the message writing space there could be an inner box inviting you to write the number of the relevant post. This however may be too much of a fiddlesome ask and in any case, you might be referring to more than one, so how many inner boxes would you put in....?
Anyway, does this make sense and if so does anyone have any thoughts on it (other than Sarah clearly has too much time on her hands!)?