Long-term (1998-present) user here. Will try to answer your questions.
> What do long-term Slashdotters think of the changes over time?
Due to the stagnation of Slashdot (going from 1,000+ comments on popular stories to 20-50), Reddit seems to have captured much of the Slashdot audience by offering a more user-friendly system, as well as hyper-specialized subreddits that address every area covered by Slashdot. Example: Reddit lets any user post a story in almost any subreddit, without moderation. That gives a more "raw and uncut" experience but also gives an organic way of seeing what other people think are hot stories. On Slashdot, the Firehose is useful and seems like an attempt at the same functionality, but it's not the same.
> How would you make Slashdot better?
I would democratize editorial choices on the front page and also make the front page non-linear. The front page is too tied to chronological ordering and not enough based on what is actually popular.
> Was ending what made Slashdot famous in favor of SEO-centric content readily available everywhere else a good thing?
No, that's what made Slashdot great.
> Would you prefer the genuinely techy "News for nerds, stuff that matters" character of Slashdot restored? Do you like it better as a generic news site or something else?
Yes, it is much better as a news for nerds website than trying to be everything to all people.
> What's your opinion of editorial quality in recent years vs pre-2012 Slashdot?
Poor.
> What would you like to tell the owners?
Bring back the old Slashdot.
> Please post at length because most current users have little or no idea of what Slashdot once was.
That's sad. This website is a shell of its former self. I wish I could do something other than write this comment.