I had an interesting idea for a new mode for survival multiplayer:
The first time a player logs into the server, his spawn point is randomly set. It is programmed to be set a considerable distance from other players spawn points, and a shorter, but still reasonable, distance from chunks that have been built in. Chat will only work short-range, so new players will not be able to talk with others from the get-go.
This mode will start very much like survival single-player, with the player being alone and having to fend for himself, but as the game progresses, and the player explores, he will find other construction and meet other people. On servers that have been around longer, the player might even find towns set up by groups of players who have already met.
The purpose of this mode is to provide a more varied experience, containing the appeal of both singleplayer and multiplayer.
I don't see any problems with this idea that aren't already a problem with multiplayer in general [people who go around and destroy everything you've done for the fun of it]. I'd play it personally, and if you can have a whole bunch of people on one server it would be interesting to see how it plays out. Perhaps we will see groups of people who start nearby banding together into 'tribes.' We may even see these tribes building towns and cities, and fighting with each other over resources. You never know.