My own four cents:
Visually this is a noticeable improvement. Cleaner design. Larger body text with reasonable max-width. Less UI "noise". I haven't messed around much with the interface; there are probably other positives. Only significant visual regression is the lack of alternating colors on comments.
(a few people have noted that the new visual scheme isn't all that site-distinctive; I'm not sure if I like it that way or not)
I see posts from names I haven't seen on LW proper in a while, which is a good sign. Content is king.
That said, some preferences the new site violates:
Lag. I get it mostly when scrolling or doing text input, rather than loading. I am not sure how much of that is the site's fault or my machine. I am tempted to blame it on the site, because...
Overly dynamic interface. I don't know what your performance bottlenecks are (I assume you're profiling and know better than me), but I see elements that could easily be static sidebars turned into foldouts, or the popup formatting overlay, or whatever it is that's making the titles on my tabs change, and...well, it smells like lagbait. Code that doesn't run can't cause performance problems, and none of that stuff is necessary. (also I don't like it stylistically, but I admit my tastes are technologically ascetic.)
Speaking of the formatting overlay: I see no way to compose in plain text with markup...any form of markup. I favor markdown, but the specific format is irrelevant, any of the modern LMLs will do. I want to not be fighting the editor (right now I'm fighting the list detection and undo), if I am thinking about the editor I am not thinking about my post, and I want the option of composing nontrivial posts in my editor of choice. If I can't copy-paste marked-up text into the posting interface, it will be either enraging (if I use it) or useless (if I don't).
There should be no topbar or bottombar, at least not on widescreen displays. I appreciate that the topbar here isn't nearly as obstructive as many other sites. I can actually see the body text without scrolling first! But horizontal space is practically free and vertical space is priceless. UI chrome should eat the former, not the latter.
Sometimes you have to clear rubble before you can build, and I know that's what this project is all about, so I'm not complaining too hard about regressions. The only dealbreaker for me right now is the editor. If I can't compose in plain text without interference, I probably won't be posting at all.
Re: dynamism and lag, one thing that really makes me suspicious is the cycling of the tab title. Something is looping in the background after the page is done loading. I'm no web developer (I do python cantrips, mostly) but I might poke around myself. What do you use for profiling?
By topbar I think I mean the app bar. The one across the top. I would prefer if, on widescreens, it were placed along the side (where the foldout menu appears, perhaps) in order to maximize available vertical space. That's an aesthetic preference, though. I do really li
I thought that instead of everyone having to create a separate post for their first impressions, it would be more convenient to create a single post for this discussion. I'll post my own here soon.