I don't really remember that much cooperation during leveling back in the day, apart from the odd group quest. It was mostly just a solo experience, but it was fun seeing the same handful of people over multiple days as they leveled in the same areas as you, since leveling actually took more than a day. I actually met some of those same people in future raiding guilds and we remembered each other. Nothing like that would happen these days.
End game was totally different. My first experience to end game was early WotLK, before any sort of LFD or CRZ. I met a lot of people on my server gearing up through 5 mans and pugging raids. Even after I joined a raiding guild I kept a lot of people on my friends list that I interacted with on a regular basis, and I even recruited some of them to guilds. Once LFD hit that tapered off and I only started interacting with my guild. There was still some raid pugging going on, but that was mostly on alts since my main was in one of the better guilds on the server.