I would fall into bucket (1), but right now it seems optimized for (3). As an entrepreneur, I'm primarily interested in seeing what other people have to say about the document, especially folks in (2) and (3). I want to know where and how it differs from a usual note, to whom those difference matters, and why it matters. I expect the bulk of the comments to be coming from investors and attorneys since they see so many more documents like this on a weekly basis than entrepreneurs, which means I want to see what they have to say.
As I was using the site, I noticed myself doing a few things. First, I was primarily looking to see where other people commented and what they had to say. Second, whenever I saw someone had made a comment, I'd stop and read — especially if it was someone I knew. Third, when I was done reading the comments, I'd want to scroll to the next one.
The overall experience for someone who wants to do these things is a little frustrating. For example, when I expand a comment section all the other sections disappear until I close it again. This tripped me up at least 3-4 times, where I'd start scrolling, realize everything else was hidden, scroll back up, and hit close. Think about it: if there aren't any other "expand section" links when I expand my first section then there's no way for me to know the difference between a paragraph with no comments and a paragraph with comments but where the "expand section" link is hidden. Indeed, in that scenario I don't even know the latter exists! What's worse, if the comment section is particularly long, like the very first comment section in this document, I'll be dumped into some random place in the document when I hit close and suddenly see before me a bunch of "expand comment" buttons. The first thing I do? Scroll all the way back up to the top to find the first one after the comments I just finished reading.
I can think of several small tweaks that might aid this. First, put little marks in the scrollbar on the left indicating where the comment section is. Second, if someone scrolls past a paragraph while a comment section is open, automatically close the other section. Third, try to find some way to allow the "expanding" behavior without hiding the other "expand section" link. I realize this is a little tricky since in theory the comment section could take up more vertical space than the paragraph it's commenting on, so the obvious solutions would require the comment sections to overlap, add space between the paragraphs in the document, or insert a scroll bar inside the section. Fourth, allow some kind of "next/previous section" navigation, using either a pair of buttons/links somewhere on the page or a keyboard shortcut, e.g., left/right arrow.
Anyhow, those are just my off-the-cuff remarks! I think the product is really neat and I wish I had annotations like this for contracts where these concerns had to be spelled out to me over time by many parties.