Second Life
Second life is by far the most user influenced game that I have played as of yet. It is essentially an area ripe for user based input. One of the most fundamental defining feature of a game is its architecture. As Second Life is almost entirely built by the players themselves, there is a lot to be seen.
To begin with, one of the most basic elements of Second Life is the world itself. In some ways, it reminds me a bit of building new house in a neighborhood. There are rules and regulations that you follow in your building process. Like in real life, you can only build as much as you can afford. As well, unless you are a master carpenter or run your own furniture store, chances are that you'll want to buy some furniture.
I once heard that the internet is great because the playing field is equal between people of higher and lower social economic status. In the world of Second Life, however, I'm not sure this is entirely true. From what I've gathered from my short stay, those people with the most money can afford to make the fanciest buildings, design the best looking avatar, and own the most land. If this is true, then it suggests there might be a deeper rift after all.
To be honest, however, I'm not entirely sure how accurate that generalization is. While it may exist in some aspects of Second Life, the differences are likely not extreme.
In general, I think my only real complaint about Second Life is just the amount of time that the world takes to rez. Lag is, after all, one of the biggest complaints in the online world, Second Life is definitely not an exception. Once the world finally loads, its a fun place to explore. I particularly liked the Numbakulla Island Project. More than not, I'm starting to get curious about the game Uru. Its interesting to see how much influence that game has had on the imagination of its players.
Emergence, however, does fascinate me. In Second Life, it can be said that emergence is basically the entire driving force of the world. Creativity is encouraged. Unlike a story that has a definitive, however, emergence has a tendency to keep on going. If there's been one case of it, then you can be sure that that won't be the end of it. Another interesting facet of the user based world is that everything is essentially a work in progress. This can be both good and bad. It is good in the sense that it allows for a great amount of diversity and branching out from the original ideas. Many new fun activities may be created, none of which were likely planned. However, it can also be bad in the sense that there doesn't need to be any sort of connection to the game and may also lack order. It doesn't necessarily have to follow any specific rules and can go any direction it wants. Once emergence starts to occur, a game will never be the same. Whether it is good or bad is up for the players to decide.