[detaro]

Deep threads hide the reply button temporarily as an anti-flamewar thing (You can still reply by clicking on the timestamp to go to the comments permalink, it's just intended to be a soft deterrent against too much back-and-forth). Purely downvotes shouldn't disable replies, only if a comment gets killed (by flags or automated filters) it gets disabled.

[domador]

So, it's not related to whether a comment is grayed out, but to how deep it is and how recently it was posted. That's helpful to know. Thanks!