I'm kind of thinking that it was more about the new owners than about the site redesign.

I still visit /., but it just isn't what it used to be. If anything, the trolls are even more vile than they were back then.

The UI changes weren't especially popular, but I don't think they were the cause of a mass exodus. I think it's more that Reddit was gaining in popularity. Slashdot posts had to go through their "editors," who were kind of laughingstocks who regularly let dumb mistakes through, reposted old articles, and regularly posted low-quality, flamebaity articles and thinly disguised advertisements. People put up with it since the articles were largely an excuse for a discussion topic. But then Reddit bypassed this and let people submit things directly. And the quality of Reddit posts and comments was a lot higher since it drew a more cerebral crowd in those early days, which made it more of a threat to Slashdot than the more mainstream Digg (which it also replaced ultimately).