Cool stuff.

You should adapt the scrolling speed to the number of characters in each comment if you're able. It's hard to do but it'll improve things tremendously.

Full disclosure, I'm waging a war against your bookmarklet's scrolling effect. See: http://www.magicscroll.net/ScrollTheWeb.html

You can try the autoscroll feature at the bottom right on that page to see how much of difference matching the scroll speed to character count makes.

My solution doesn't work well with tree structured documents like comment threads though. Perhaps I need to adapt it.