Actually, guestbook entries were typed on a separate page. They weren't scribbles on the main site itself. That separate page is what made it feel different.
Man, you hit on something there. It was like a little message board just for your site. People would leave these random stories or inside jokes that only made sense if you'd read the whole site. I remember someone wrote a whole paragraph about how my midi file of the X-Files theme scared their cat. You don't get that kind of effort with a like button.
Know that separate page made all the difference. It was a dedicated space for words, not just part of the site. That's why it felt like real conversation.