somewhat close to what lone wolf said, I consider it the addition of the resources, then the addition of the cauldron to use these resources up to make new 'objects'.
this is how I'm thinking : the resources have lead to people collecting (not sure if wood was there before stage 10...) and the collecting to a new market item and new trade, the stones in particular have lead to newer players being able to cast magic and enjoy it's joy, it also is almost the only thing that has lead to the appearance of the fusionairs , also the water resources has lead to a guild being formed...
and as for the cauldron, even though I know that many people have the ability to make items in MD, what this cauldron makes is not items, it makes.... basically, it makes anything, can do anything, and all it requires is a few resources, this cauldron makes MD even more free as it introduces a 'make whatever you want yourself' system, and so, these changes to MD, it's market, it's game play, it's players, it's guilds, and it's objects are all vast enough to make it go to the next stage
as for the viscosity, it is a big change, but changing the way of how people go from place to place is not exactly a HUGE change, sure, it has massive effects on exploring, but it doesn't affect ALL of MD directly, and so I don't think it could be the change Mur meant =)