I share the author's feeling that World of Warcraft (WoW) developed a strong sense of "place" in my psyche. I also think his methods quite unscientific :-) but share the curiosity of the question.

One of the things that gave WoW its appeal was that there was a lot going on in the environment. Animals scurry, there are houses with belongings in them, ruins, and various natural formations. Basically a lot of "cool" stuff to look at while you were looking for your target monster or next quest giver.

Of course WoW had different servers with the same environment but different play styles. The big separator was "Play vs. Environment" and "Player vs. Player."

In the former, none of the other players could attack you unless you "enabled" player vs player, or "flagged" (entered a zone that automatically enabled it like a capitol city, or manually set it and then hit someone who had also set it.) The effect of that choice was that the environment was fairly consistent with the story line that was being enacted. (yes sometimes opposing factions killed of quest givers and that was annoying but the mount of "grief" they could cause had limits).

Contrast that to player vs. player servers where entire zones were "owned" by one faction or another and characters were always vulnerable to attack (although not from players in your own faction outside the nominal 'dueling' capability). These servers could be much more difficult to play on, especially when the populations of the two factions were unbalanced in favor of one faction or the other.

Out of necessity, the quest giving non-player characters (NPCs) were typically either stationary or covered a very fixed path. As a result when repeating zones on different characters it could feel both 'boring' and somehow 'reassuring' when you went from one quest to the next.

The ease of setting up groups to quest together (co-operative play) and the social aspects of that gave the time spent a lot of "memorable" qualities. The relative challenge (especially in the first or 'vanilla' version) of the quests gave a sense of accomplishment when multiplayer quests were completed.

There was a fixed "script" in terms of how the mobs would behave, but players could be quite creative when it came to how to overcome those scripts. That also made play more memorable.

Finally, and the author touches on this, the large number of choices and NPC factions give individual players many choices on how they gained experience to get to the maximum level. For people like me, the "mini" games, like crafting, the auction house market dynamics, cooking and fishing made the world seem so much more "real" than in other games.

Having things to do in game that were not "required" for advancing, but could change how the game felt (like eating something you cooked which would change your player stats in some beneficial way) contributed to this as well.

Finally, the openness, and sheer size of the environment really encouraged exploration. The difference between running/walking, riding a mount (slow or fast), and flying (faster, and faster still) was really well done. While it could feel bleak at times Blizzard did an excellent job of providing story and nuance around the bleakness.

I've said in the past but Microsoft could probably pretty easily "dominate" the whole metaverse thing by pushing out the WoW vanilla engine as a community environment. One of my old guilds would have meetings in a building in Ironforge (a capital city), we could all come together and sit on benches and talk about the news of the day and make plans for things. Normalize the in game economy, create things that wear out to insure turnover etc and you could easily have an endless world of friends hanging out doing interesting things.