- reducing the amount of value in the economy via means such as: item decay, item deletion (either via challenges where you can lose items, or manually; yes, some games literally just go in and delete items), character purges, etc. Many games used to simply wipe the player database every few months because mudflation had gotten so bad.
- Refusing to up the level limit, and instead introducing orthogonal advancement paths. This single mechanism is probably the single biggest slowdown you can effect. Many long-running successful muds got that way by simply never upping the level cap, and instead investing their expansion in enriching the content and encouraging repeat play.
- Obsessive attention to economic stats and adjusting all economic drains to account for all the influx. This often means punishingly high costs for players, btw.
- Never introducing higher DPS mechanisms; in other words, the levels become cosmetic, because the actual power of players does not increase. This is how the systems with “infinite levels” tend to work — they asymptotically approach zero player power growth.
- Shifting player attention from power gains to cosmetic gains (e.g., instead of a new badass sword, gain the ability to redesign your sword and personalize it).
- Radically altered elder games (politics, economics, PvP, etc). This runs the risk of alienating players who liked the game they were already playing.
- Remort systems and other such mechanisms to encourage repeat play of lower levels.
- Restrictions on trade, such as soulbinding items, so they cannot be handed down and thus increase the ’standard of living” for all users.
- Level limits on equipment use, so that you are forced to abandon equipment (which in a soulbinding game, effectively means item deletion).
It would help if MUD items gave fixed bonuses instead of percentage increases. A +20% sword is always good but a +1 sword is only nice at lower levels. Of course, if the rewards are other than killing things and taking there stuff then you don't need to worry about levels much at all.
How about a wuffle based system? Wuffle is Cory Doctorow's word for social capital as defined in Down & Out in the Magic Kingdom (which is an OK read btw although the characters are annoyingly adolescent - but I guess that's what you get in societies where no one has to work, the Culture is the same which is why the best characters are limnal in some way).
I think abilities could be capped in a zero based way (Social skills + Physical prowess + Craft and Magic = c but start out at less than c). That way you can improve your character, up to a maximum but specilisation would come at the detriment of other skills. Wuffle on the other hand would give access to better interaction with NPCs, perhaps property or even titles. Wuffle could even be aspected so that you might have different Wuffle from different kingdoms.