Our server first Ossirian kill in AQ20. We messed up on the last crystal, and he started wiping the raid while at sub-1% health. I felt the urge to reincarnate, casted a lightning bolt and an Earth Shock, and he died mere steps away from me. I just can't forget that
Teldrassil specifically is a quite nostalgic place for me since it was so beautiful to look at. It's also the first place I actively leveled through, playing a nelf druid and hunter.
Elwynn Forest is nostalgic for me because many people used to hang out in Goldshire back on my old server.
Dun Morogh is nostalgic because it was the place where I first started my WoW adventure playing a gnome rogue.
As for cities; SW, Ironforge and Darnassus are pretty nostalgic for me.
My ever first world pvp raid from stormwind to barren during open beta in Nov. 2004, when most people including me at the time being very exciting didn't really know what was coming. That was still one of the most nostalgic moments to me after 9 years.