In this particular mock it's slightly confusing to me that the left nav looks like it has a "Table of Contents" header but is showing a document hierarchy (i.e., element that looks like a header does not seem to be labeling the content beneath it). Not sure if that's a mock thing / interaction thing / whatever.

I generally like this direction and un-burying / granting prominence to TOC/hierarchy, though.

Hah, I actually had the page title instead of table of contents but it felt really repetitive

This it exciting, @chad and mostly I like the look of it.

A few things stand out to me here.

• It took me a while to realise what was in the left sidebar. I assumed it would be an outline of the document, but it's actually a list of related documents. I see others noticed this too. I don't think 'Table Of Contents' as a heading fixes this (I would still think it was a document outline). Perhaps something like 'Related Pages' would, or 'Navigation' as you've called it elsewhere.
• However, I have found intra-document navigation more difficult since the outline was moved into the menu. If there can be a way to bring it back out somehow, that would be nice. We tend to write fewer, longer documents, rather than many documents that need to be navigated.
• That said, we have some pages that are primarily for navigation, sort of like section pages. Perhaps a per-document setting for the left sidebar to either show the document outline or the related pages would be good (and allow the user to switch to the other, perhaps via a tab interface). Then the most appropriate display can be set per-page.
• I love that the 'related pages' navigation is customisable. I see some discussion of this being able to be shared among pages, which I think is essential. It's a lot better than the current automatic subpages display (I have one page on our wiki with subpages listed in the order they are intended to be read, right above the automatic list of subpages in alphabetical order).
• There doesn't seem to be any Remarkup facility for comments. That would be pretty useless as soon as we want to discuss anything technical. Something more lightweight, i.e. more like Conpherence and less like Maniphest would be OK, but I think there needs to be Remarkup support there.
• I think it would also be important for the comments to scroll independently from the page content, so you can refer to the part of the page that any particular comment is about. Without that, it's easier to do as we currently do, and comment in a Maniphest task and use multiple browser tabs or windows.
• You didn't mock 'Timeline' but I guess that's like the timeline in other apps. I wonder if it could/should be combined with 'History'. They seem like synonyms to me and I can imagine them easily getting confused for some minor annoyance.

The comment area takes remarkup. I think every textarea in Phabricator
takes remarkup, regardless of if it has the hint bar or not.

Cool. Well that clears up one concern right away. :-D