One of the first things I do in any MMO is a test to see whether I can sit in a chair in the world. Not just emote that I'm sitting, but walk up to a chair, click it and sit down.
It's something that doesn't always happen in an MMO's world, and it's a sign of a world that is going to feel more interactive than one that is stagnant. To me, this is important. That extra layer of interactivity makes the world even more alive than just the stories and people inhabiting it. I hate that in some MMOs, for me to sit in a chair, I have to stand on it and then use the "sit" emote. And even then, it doesn't always work out.
This is just one area where I feel a game world can come alive, the world's level of interactivity with the player. In my opinion, you should be able to affect and use the objects in a game world in a realistic way.
But it's more than that for me as well. Believable characters that fill the world go a long way towards making it feel real and alive. Towns that aren't just filled with players but NPCs going about their business help to sell this illusion. Walking into a dense forest and not just seeing enemies but hearing them as part of the soundscape puts me in that world.
I also love worlds that have more things to do than just quests. One of my favorite things in The Lord of the Rings Online is to just sit in my Kinship house with friends, talking on Discord and drinking from our keg, waiting to see where the world is going to plunk us down when we over imbibe. The Elder Scrolls Online's Antiquities system, while not my favorite thing they've added, offers a deeper dive into Tamriel's lore and a way to earn rewards outside just killing baddies. The upcoming card game Tales of Tribute in High Isle aims to flesh this out even more by just giving you more to do that adds to the world.
So for you, what about your favorite MMO that makes the world feel alive? Is it a location? Stories being told in-game? Or is the atmosphere of an area? Let us know in the comments.