Mine would also be from FFXI:
Most of them are little things, like with it being my first MMO, even just walking through towns and thinking "Wow, all of these characters are REAL PEOPLE" it just blew my mind.

I also enjoyed other little touches from XI before it got heavily nerfed later on, such as monsters not only relentlessly chasing players across an entire zone, but also remaining next to the Zone even if it was a town until someone took care of it. I used to always see actual players standing guard in front of the three main towns to protect newbies from this. It was just so cool to me.