Lurker showing up just to add my own voice.
While I don't like the trend towards 'sleek' and 'modern' designs for anything -- be it company logos or websites or what-have-you -- I can generally tolerate the design so long as it's only a visual update. For example, I do actually really like the super dark background (though I get why it would strain some folks). This is how I tend to style my own desktop.
But it's not just a reskinned UI with new colors and slightly rearranged elements. I don't want to rehash what others have said -- search mechanics, so much empty space, lag, etc -- but I do agree with much of what has been said here.
Honestly, I would even be happy with even just a way to opt into a format like the old UI but keeping the colors/style of the new. That's really just my main gripe. It's not the idea of change (okay, I will willingly admit the need to make everything sleek and shiny is a design pet peeve of mine, but even then) so much as the fact that everything feels a lot more inconvenient. And not just due to it being new.
It's a real shame that I dislike using the site and the new layout so much. I used to just have fun browsing mods for games, a bit like a modder's version of windowshopping, ooh maybe I'll get a little something for myself, but I have no desire to do that now. I log in, see if there's a specific mod for what I seek (ex. QoL on some game, bg3 dragonborn faces, whatever) and if not, I leave.
I am a casual user. I like downloading files like i'm playing with a new toy. But I can't see myself browsing through the site like I used to. Browsing, sure, I don't doubt it. But enthusiastically going through 40 pages for an hour or two? Not at all. I do not want to spend more time on nexus than is absolutely necessary, now.