You could make things exist in the world pretty much by describing them, and build parts of the game that were essentially your own subgame, and do this using your basic language skills. I had tried and bounced off some early 3d game-making experiences due to their low quality-to-effort ratio, but building experiences with MUDs (whether that be RP experiences or worlds or bots) was pretty immediately rewarding.
By extension, this is related to tabletop roleplaying as well. Problem is that there was too much missing from MUDs compared to TRPG that MUDs fell by the wayside. There are plenty of reasons but I think the most significant is that it takes too long to type things.
I really liked the idea of an authored multiplayer experience. I played a MOO where game masters could open up a character's bio and listen to any thoughts the player said with the 'think' command and set up scenarios. Of course, players would do plenty of roleplaying just with themselves when GMs weren't around, but it meant that a GM could lead players to each other, or create events through prophecies. There's something really exciting about a D&D-like experience where it really feels like you're in a world which isn't just revolving around you.
It's difficult to do much when typing takes so long too. Most MUDs I played also didn't have any indication if another player was typing or not, which definitely didn't help. It's probably wishful thinking, but if text-to-speech ever becomes available, that could make things much simpler. On the other hand, we usually hold writing up to a much higher standard than what is typically found in a TRPG.