I easily get pages into a post before deciding I just don’t care about it all that much and just don’t bother submitting it.

I do the exact same thing - writing a wall of text, just to close the window and never submit it!

There’s several things social media sites could do in order to cut down on fluff and encourage more engaging posts, but it means walking away from the easy karma models.

Fully agree with this. And Slashdot itself did a few things right in this direction, like the +5 karma/post limit that you mentioned or acknowledging that people upvote things for different reasons (funny, insightful, etc.). It's just that certain businesses like Reddit don't really care about quality of the experience of its users - they care about ad views per second.