I'm having a really hard time with this one. It's not that I don't have great memories of playing MMOs, WoW in particular; but many times, it's more the general feelings associated with those games that are great. It doesn't help that I am currently working longer hours, so my brain is a little befuddled.
When I say feelings, I mean like how it feels to fly in my STO starships, whether interstellar space or in battle around a planet. It's just fun and freeing; I pan around my ship looking for awesome shots, like the one in the blog masthead above.
Or how it felt the first time I realized I was powerful in WoW. Rowanblaze the priest was in in the Hinterlands at around level 40, and multiple wolves of about my level attacked. I defeated them all and survived. It was great.
The opening crawl of MY story during beta in SWTOR was exhilarating. I was so excited to discover the adventures that lay ahead. I was also excited to hit 50 in Rift together with my lovely bride a few weeks after we were married. It was something we'd accomplished together.
Many of of my best memories were of playing with good friends, like running through lowbie areas on brand-new characters with friends I'd met in Germany. Or introducing a girlfriend or two to the genre and playing with them. Chatting with guildies as I taxied from one zone to another. Downing the final boss in Kharazan, really many raids where we could joke with each other and cement friendships.
I think that's the greatest thing that MMOs can still do, is bring people together across great physical distances to laugh and have some fun together, whether it's an endgame raid, or a little PvP, or just playing roles in some remote tavern on Telara, Kalimdor or Tatooine.