f20 selects this discussion (the 20th link), but I used f4 to select threads (the 4th link); j,D, and scroll down by a line, half-screen/page and full-screen/page respectively; fr4 selects the correct reply (the fourth link whose text begins with r); gi goes to the first input field and enters insert mode.
In insert mode I may use the simple mixture of emacs/readline and CUA bindings — like ^w to wipe the last word, ^a to go to the beggining of the line and ^e to go to the end (eg. when inserting a > before a blockquote, then appending my commentary) — to write a straightforward comment from beginning to end, or I may use ^i to pop the field open in my $EDITOR (vis), where motions like m to mark my place, G to go to the end, o to open a new line, and M to return to my mark are useful for comments that might be written non-linearly, eg. if they have footnotes. I can save and exit vis (if I use it) with ZZ, and submit the comment with standard CUA .
If I reference any other material then Pentadactyl’s browsing bindings like go«character» to open a quickmark, s to search, p or P to paste my X11 primary selection as a search term or the url of a current or new tab, b to fast-as-you-type search through tabs, y to yank the current website, ;y«number and/or title of link to yank» and d to delete a tab are useful (along with standard X11 Shift+Insert to paste the primary selection in insert mode).
Ideally though, I should use Pentadactyl only for browsing — for scrolling through and reading web documents — and have a dedicated newsreader that understands what posts, comments, etc. are and could make this several times more efficient. I’m currently dipping my toes in evil-mode emacs, which should enable this. I’ve recently spent less time on HN and more on IRC where I do have a dedicated native client that makes things orders of magnitude more smooth than trying to use a web client in my browser.