A few mostly-superficial first impressions:
1. When I go to my profile page, among the things there is a selection of ten "recent comments". These range from 7 months ago to 8 years ago. The older ones aren't super-highly-voted-for or anything like that. Even if the import from LW 1.0 has pulled Main but not Discussion material, which would certainly lose a lot of my comments, there is something really weird with the selection here. There's also no way to choose how those comments are sorted, which might be OK if it were something simple and clear (oldest first, newest first, highest-voted first) but actually it seems to be pretty much random.
2. There's some indication on my profile page that I have 1 of something, 23 of something else, and 5306 of something else. There are no explanatory words or hover-text or anything. I guess the last one is comments, the first one (with a star) might be "favourites" in some sense, and perhaps the middle one is posts. But there's no way to see whatever posts there may be. (My profile has a "Blog posts" section but it's empty. I don't know whether that refers to posts on LW, of which I definitely have some both in Main and in Discussion, or to imported/linked blog posts from elsewhere, of which indeed I have none.) Surely, from a profile page there ought to be a way to see the user's posts.
3. Clearly we have preserved votes on comments from old-LW. Perhaps that means users have karma scores. But I can't see mine anywhere. Maybe that's a good thing, of course. And maybe the details of how -- if at all -- to aggregate votes on posts and comment to get per-user karma/reputation are still being hashed out; but it seems like this will affect the dynamics of the community, and we don't have that long before we have to decide whether to stick with this thing or throw it away...
4. Formatting of quoted material in comments is weird: it comes out larger than everything else, with extra vertical space below but not above. (At least one one of my computer/browser combinations: recent-ish Firefox on Unix.)
5. I am with those who don't much like the look of the text. It's better on higher-resolution displays but far from delightful anywhere.
6. As I enter more text into the comment editor, the region occupied by my text extends downwards. After a while it overlaps, and sits behind, the dark bar with the formatting buttons on it and becomes almost, but not quite, invisible. Surely this can't be the optimal behaviour.
7. Someone else asked for a RSS feed for posts (I guess Atom would do just as well). I second this request. I haven't looked to see whether there are feeds for anything else; I would encourage a general policy of having feeds for everything anyone might want, if performance considerations allow.
8. Following a link to an old comment is a frustrating experience, at least sometimes. You get this weird page that has the comment you already read at the top of the page, without any context. Then there's a "comment in full context" link (if I didn't want the comment in full context, I wouldn't have come to the page in the first place), except that it usually doesn't work because only 50 comments have been loaded. So then you have to load more comments. 50 at a time, just because. Taking a substantial amount of time for each block of 50 to load. Only then can you follow that full-context link and have it actually work.
[EDITED to add more:]
9. Clicking "Reply" opens up a comment not-box ... and doesn't give it focus. That's really annoying.
10. "LESSWRONG" in the bar at the top of each page seems like it ought to be a link, but it isn't. It's a thing you can click on that navigates your current window to the front page. You can't middle-click on it to open a new tab. You can't right-click to get options like "Open in new tab/window". So far as I can see, there is no benefit at all to this over just making it be a link. Please just make it be a link.
11. On old-LW every page has a right-hand sidebar containing lots of things I don't care about, and some things I do like to have at my fingertips like the list of most recent posts and comments. Here there's nothing of the kind: there's just a single "middle" column with article + comments or whatever. For me, this -- perhaps in combination with the mostly-grey presentation -- has a curious psychological effect: somehow it doesn't feel as if I'm on a website but more as if I'm reading a book or technical paper or something. And, perhaps because this is not in fact a book or technical paper and doesn't have pages or chapter divisions or anything, the net effect is that when reading a lot of comments here I feel like I'm adrift in a trackless waste with no signposts to guide me :-). I am only guessing at the elements that contribute to this feeling, but for whatever reason I don't get it so much on old-LW or even in SSC's monstrous comment sections.
[EDITED again to add even more:]
12. Headings in comments are, or at least sometimes are, really large. Comments that use them can end up with more prominence than the article they're commenting on, and the rather uniform-looking design of everything here can make it hard to see at a glance what's article and what's comment. This is especially bad when the comment in question is the one being shown at the top of a page because you followed a link-to-a-comment; this is a particularly clear (or, in other words, particularly unclear) example.
13. Timestamps are shown as "5h", "6d", "7mo", etc., indicating roughly how long ago a thing was posted. That's nice and friendly. But it would be nice to be able to get more precision, especially when viewing comments sorted in some way other than chronologically. In an ideal world, (1) hovering over the timestamp would show the actual date+time and (2) there would be a per-user preference setting that would reverse this, so that you get the date+time by default and can hover for a user-friendly about-how-long-ago view.
14. Comments have net scores displayed but no indication of e.g. whether +1 means "one person liked it" or "101 people liked it and 100 people hated it". Again, it would be nice if hovering over the score said something like "+17-15". (Not, please, the "percentage positive" we have on old-LW.)