I understand why it's nice - esp. on mobile phones - to have posts no longer than* X* length. Browsability matters!
However, limiting the default characters significantly limits #mastodon adoption, as it is inappropriate for what they want to do... which could be everything from #journalism to #writing to #fanfiction.

But devs solved this problem 25 years ago on #livejournal, with the lj-cut tag.
Hey, @Gargron ... a time-tested #opensource solution that we KNOW works!
https://www.livejournal.com/support/faq/75.html