Clicking on "[add /] show n more comments" grabs the focus, placing it in the newly revealed add comment field. This prevents cursor keys from scrolling, which is driving me up the wall. Probably only significant if using a non-Apple trackpad/touchscreen*.
If you haven't seen all the comments, I think it's inappropriate to have any kind of commitment to adding a comment. So perhaps the "add /" could be removed, with the usual "add comment" appearing once expanded.
(*My Mac screen is broken, so now using a touchscreen netbook [rubberised Latitude 2100] with standard bad PC trackpad (two-finger scrolling is vomit inducing).)