[josquindesprez]

I imagine for popular posts, you'd get way too many low-value bumps: lots of leaves in the comment tree would be low-effort back and forth replies of uninteresting content (e.g. Reddit). If there's a voting system, the top-level thread would get bumped a lot but it'd be difficult to find that low-value content. The signal that the bump is trying to convey (there's new and good stuff here!) wouldn't match the value of the information that the user gets (the scattered dregs of many conversations). If there isn't voting and you sort by newest content, the experience would probably be like 4chan in slow motion: bump ordered blasts of low-value replies.

In a classic forum, since posting something not only moves the thread to the top, but puts your post in a highly visible place (the end of the thread), it's a lot harder to have too many deeply branching side conversations with low value replies: there's a social norm against polluting threads.

For something like HN where comment trees don't get too deep and the replies are always meaningful, I think threaded bump ordered ranking could work.

[heartbeats]

No, posting somewhere moves the whole subthread to the top.

Before:

  ├── a
  │   ├── b
  │   │   ├── c
  │   │   └── d
  │   └── e  
  └── f
      ├── g
      │   ├── h
      │   └── *i* <- this is new
      └── j

After:

  ├── f
  │   ├── g
  │   │   ├── i <- moved to top of its thread and its thread moved to the top
  │   │   └── h
  │   └── j  
  └── a
      ├── b
      │   ├── c
      │   └── d
      └── e