... is, in my opinion, the Note Poles.
Thank you very much, TS!
Since Note Poles were implemented, the whole game got a big boost. People are actually creating things on their own, explaining, describing, getting vividly imaginative. The museums in the towers are just one fine example for that. I just had a lot of fun hopping from one tower to the other and look inside.
Until now, the home streets felt a bit detached from Ur. As if Ur and the homes were two different things. Now I really start to feel that both parts blend into each other: It is as much fun to wander around home streets and discover them, as it was fun to walk around Ur "before Beta 2.0".