I miss the variety of old fashioned forums like this one. There's barely any left and just about all of them are nearly dead.
I also miss my cringe days in various fandoms; there was something about the participation in the community surrounding something I loved that really struck a cord with me. I loved reading other people's take on fandom lore and characters, and the intense speculation that used to arise when everyone was waiting for the next installment of insert-geekdom-here, all the debate and anticipation was delicious. Now every fucking character is a tranny, and fandom discussion is largely controlled by the media owner.
I miss the heyday of LiveJournal; not just for geek shit, but for the friends I had there who've since moved on, and the odd communities of interesting shit.
Like
@Kari Kamiya said, I miss character shrines; a few of them are still around but they haven't been updated for literal decades, and the entire fandom has moved onto Facebook and Tumblr and Reddit and ad-infested wikis.
I miss the anything-goes approach to the internet. Now everything has the absolute fuck moderated out of it. It's not that I miss saying 'nigger', because I never actually said the word before people told me I couldn't. In the wild days of the internet, places that enforced a code of conduct did so because it was good manners and made for a harmonious community. Now, absolutely every single letter, numeral and punctuation mark is rigorously scanned for the vague scent of insert-phobia-here. Mere mild statements of a different point of view will get you kicked off multiple sites because invariably, some snowflake will feel 'unsafe' because you questioned their cherished reality. I miss being able to tell people that they were full of shit and need to toughen the fuck up. I miss being able to have a mild debate without being called an "-ist", or even being able to say, "I still disagree, but I respect the way you defend your position." In so many places now silence is a crime, because if you don't loudly support the current thing, well, "silence is violence."
I miss lesbian communities that aren't filled with troons or straight girls calling themselves 'queer'.
I miss not tripping over some underemployed wanker proclaiming themselves to be some kind of 'activist' every second paragraph.
I miss little random websites of... well, random.
I miss 4chan when it was good (but then 4chan was never good.)