Students at my college hate having to scroll for ages within long discussion topics, just to be able to read the latest post (they prefer keeping the display set to flat with oldest first). This is also a very bad thing in terms of page loading times, i.e. pages with many posts take too long to load.
I suggest that an option is added that allows you to specify a max number of posts per page, in the same way you can specify the number of discussion topic per page. This would minimise the amount of scrolling users have to do and would make forums more user friendly.
I've added this as a new feature request in the issue tracker (see MDL-10120).