Hate:
Web designs clearly built with mobile in mind, with "mobile friendly" elements everywhere even on the desktop version.
I don't need fucking inch-wide buttons when I'm clearly using a mouse and keyboard, and I fucking hate the actual content taking up the middle 1/3 of the screen while the left and right 3rds are taken up by either ads or whitespace.
Surveillance Capitalism
"We put a tracker on you a while back when you let your guard down, and we think you'll like these products. :^)" Die in a fire, Google. How the hell we let things get this bad I will never know. It's the creepiest shit, and nobody bats an eye about it. We're giving money to companies that invade our privacy and psychologically manipulate us to impulse buy shit we don't need.
"Javascript must be enabled/Adblock must be disabled to continue"
Go fuck yourselves. There's no damned reason why Javascript should ever be necessary on a modern website, especially to read a text article. Any time a website tries to strongarm me into using Javascript for a passive reading experience, I instantly distrust that site. Also, I'll disable adblock when the ads I'm blocking stop being a godawful mess that takes up 80% of the page to shill eerily relevant products (see above).
Auto playing videos in articles/on pages
Who the fuck decided this was okay? If I wanted to passively watch a video, I'd go to YouTube or something. I'm here to read an article or something, so fuck off. I honestly don't know which I hate more, the videos that are embedded somewhere on the page so they start playing after I've made it halfway down and have to scroll back up to look for the fuckers, or the ones that float in the corner and chase me down. They're always at full volume too, so god help you if you're browsing in the middle of the night with speakers near someone who's sleeping, or by yourself with headphones on.
Miss:
Flip phones.
Remember when the entire rest of your phone's components were between your screen and the ground if it fell out of your pocket? There was always something so satisfying about snapping that bitch shut, too. You could also actually fit them in your pocket and still have pocket space for other shit. I feel bad for females, with their inexplicably small pockets.
Instant Messaging.
Catching your friends when they were home, getting a little peek into someone's mood via their status messages, etc. There was a ritual to IMing that got lost with modern messaging clients, I think. It felt like a special place where you could open up a bit more to your buddies, confess your undying love for someone, etc. Now it's just another tool to be either incredibly vapid or srs bsns, with no go between.
Personal web pages.
Web pages used to be an expression of self. You could tell a lot about a person by how they designed their website. Passionate perfectionists always had an "under construction" banner despite new content being added regularly, whimsical types' pages would change your mouse cursor to something silly, etc. It also required an initial "buy in" of learning some basic HTML, so someone had to be inspired to create one. Now we Facebook, which is overly sanitary and too accessible, so the experience ranges from overly utilitarian to cringy boomer memes.
Nerd cultures.
When you had to have some technical know-how to get around on the internet, we had such an abundance of people who values intelligence and scientific thought that it couldn't even be contained within one group. You had the phreakers, the hackers, the scifi geeks, the tabletop gamers, and hell, even the furries. These groups intermingled, but they were all pretty insular and despised the idea of average people moving into their space, so there was a unique culture built around intellectualism and shared interests which were discussed at length. People would rep their favorite franchises in forum signatures/avatars, but to be part of the community you had to do more than buy a t-shirt and subscribe to a YouTube channel - you had to know what the fuck you were talking about, and contribute to the discussion in some way.
Diversity of platforms and thought.
This is probably my biggest one. There were multiple places to go and discuss any given topic, each run by a person or group of people with their own ideas on moderation, and if none of those suited your desires you could make one yourself and actually be seen. Until the late 2000s or so, you could look around and see people with opposing viewpoints having respectful, in depth debates with nary an insult. Nobody tried to play "community leader" and somehow revoke someone's rights to enjoy something because they disagreed with them, and if they did they'd be laughed out of whatever forum/chat for being a little bitch. De-platforming wasn't possible, because there was an abundance of platforms. The internet was treated as a proper commons back then, where everybody had the right to their own opinions and the ability to gather with others and form a community.