The dial up modem sounds. I don't know why, but I genuinely miss them
I also miss the reduced footprint of mega companies with their "we are the internet" monopolistic tendencies. They still wanted to be the entire internet, but they weren't.
I miss when Google's motto was "Do no evil".
I miss when Usenet was for something more than downloading porn and pirated content
I miss Geocities and everyone having their own shitty webpage
I don't miss IRC and netsplits, or images that would load line by line and rearrange your page as they did. I don't miss JavaScript popup ads or websites that played looping wav files with no easy option to stop them.