There’s a new Reply link at the bottom of each comment. If you use it, you get indented threaded commenting for things that are direct responses. Or ignore it, and just use the regular box. This is a new WordPress.com feature, so you’ll likely see it a bunch of places. I tried it out on the last post, but my comments are not such to require threading, so I may turn it off.
In announcing it, WP.com blog wrote:
To save you and your readers from scroll-finger strain, you can now break up comments into pages.
Uh, moving the mouse to click a next page link is WAY more straining than hitting the space bar. No one on a computer (as opposed to a phone) should be using finger-scroll for the web (and I wish many many other programs let me scroll by hitting the space bar, especially iPhoto and the iTunesStore. I can see that it wouldn’t work well in MS Word).