Just going on record that I would never censor myself by 'shortening' a post I felt was as long as I wanted it to be with spoiler tags or any other mechanism. I suspect there is an upper character limit on posts enforced automatically but beyond that I think the text portion of posts should aways be displayed in their entirety.

If something is off topic then it should certainly be taken to PM but if two people happen to be the only ones who choose to discuss an issue that pertains to the topic of the thread back and forth for a while I see no reason why they should have to censor themselves in case someone wants to skip their entire exchange.

An alternative, probably not viable without making the db or the site do extra work might be an option to let everyone set their own 'max post display length' and crop any posts that are longer than this length for the users who choose to do so. That way only the people who actively dislike long posts will be affected (beyond the general overhead of handling the feature of course).