Simply looking at the interactivity of MUD doesn't address the lure of these environments. After all, we interact face-to-face with others all day long in the real world. There is something else exciting about exchanges on MUDs that is harder to put a finger on. Watching another character, you notice that it describes itself singing a certain song, and you think to ask if they've heard of an obscure band you like. You type out a question and hit the return key. Your question goes out to everyone in the room, and you wait, watching actions of other characters in the meantime. You watch a minute pass, and a reply comes back on your screen: the other player loves that band too. You ask another question about music, and the online 'conversation' begins.
Perhaps the only experience that closely resembles this language event in real life is passing notes during class in grade school. You write a note on a scrap of paper, and stealthily pass it to Lucy, anxiously watching as she writes out a note in reply. The message is passed back to you and you read it with excitement, ready to send another note. Asking the same question of Lucy orally, outside of class, never seems the same as scrawling out the words as fragments on a physical object and visually handing the meaningful note back and forth.