What is the key 'gameplay loop' in a MUD game?

Hmm. Honestly I think that the thing I like most about MUDs is that there isn’t necessarily one way to play them. They can be social or solo. They can be combat-focused or story-focused or puzzle-focused. And probably most critically, you are given the option of changing how you play at any given time without needing to change characters. So I suppose the thing that makes MUDs appealing is the sense of agency.

I have sampled several MUDs in 2022 and wrote about them in several comments and one post. I am not a traditional MUD player, and I never played them back in their golden age. Many of my opinions about these games are not shared by "real" MUD players. For more traditional, and possibly more positive takes, take a look at /r/mud.

The vast majority of MUDs can be aptly described as "text-based MMORPGs". The resemblance is profound. Generally speaking, MUDs are games in which you are very concerned with the progression of your character.

That is, of course, not true for every MUD. Many have a relevant, and to, varying extents, mandatory roleplaying aspect. Some favor immersion more than anything, requiring you to be in character at all times and act in accordance with it. Exploration is certainly a major aspect of pretty much every MUD.

A MUD will generally "click" when you start to understand your character's build (like in Aardwolf MUD), when you have that one magical roleplaying experience (like in Legends of the Jedi), when you feel that the world reacts to you in ways that make it feel alive and rich ([like in Discworld MUD), or when you build connections to other players and chat becomes a new home.

None of those aspects are exclusive to MUDs, but some are more likely to happen in MUDs than elsewhere.

Other than the text-based nature, what makes MUDs unique is how they're developed and how this changes your attitudes toward them. You see, even though many MUDs will accept "donations" that grant some kind of non-pay-to-win "gift" (effectively purchases with another name, but don't tell /r/mud I said that...) the vast majority of MUDs are developed and maintained by non-professional volunteers. Most of these games were created in the 1990s, some even before that, and have been expanded and supported since then, largely by players that "graduate" into maintainers. Even openly commercial MUDs use lots of unpaid labor and are developed in a way that takes the player base's historical expectations into account. All of that means that you cannot expect MUDs to "just work" as you do with a regular commercial game.

Some things may never be fixed not due to incompetence, but rather because the ethos of mudding is that you are a bit of a developer. You must understand that engaging with the community is not optional, and if something is "broke" it is your responsibility to seek help to circumvent it, help others with the knowledge you accumulate, report a bug, or even put in a request to fix those things yourself when you are more experienced with the game. I am not saying MUDs are a buggy mess, but they're not like downloading a game on Xbox either. If you're not willing to really engage with the mechanics and systems on a closer level, expecting things to "just work", you may find yourself frustrated. You'll also have to take notes, by the way, as there won't be a neat questlog with all the details and narrative for you to look up, or an arrow on the minimap pointing exactly where you have to go (hence the exploration bit). You are pretty much on your own.

There is also the whole aspect of choosing a client and configuring it to your liking, which can be a journey in itself. Even more so because scripting is backed into the culture of the genre. While that is generally only mandatory for PvP, the way some people play can become so heavily automated that it is more like a coding challenge than "real" gameplay.

There are also different types of MUDs, with different expectations. The regular combat-focused MUDs (often DikuMUDs) are like a regular MMORPG. Roleplay Enforced (RPE) MUDs have an important roleplaying aspect. You are not required to roleplay, but there are rules you must follow in order to maintain a healthy roleplaying environment. Roleplay Intensive (RPI) MUDs require you to always be in character. They restrict or remove any out-of-character communications, and may even try to prevent you from disclosing in-character information outside of the game (like on Discord for example). RPIs often have a harsh, deadly, unforgiving vibe, but there is also a My Little Pony RPI that is the complete opposite.

Outside of MUDs, there are also MUSH (Multi-User Shared Hallucination) games, which are scene-based roleplaying with much lighter text-based mechanics that are only there to support roleplaying.