The level is generated on the client. However, the seed for generating the map is sent from the server. In this way, we generate the same world for all players, but we don't have to send the entire level. We plan on regulary changing this seed (say, once a month), so that this place changes regulary. Hopefully, this encourages players to talk about what they find!
To be perfectly exact, there's actually two seeds: One for the rough world geometry (mountains, valleys, water, that kinda stuff). And one that determines placement of objects, such as trees, rocks, etc., and - perhaps more interestingly - that defines the order of these "rooms" that you traverse the world in. It also affects the placement of dungeons and enemies (therefore, also the difficulty of enemies).
We decided for this approach so that ALL players would see the same world, roughly, but the two teams would traverse and experience it differently.
Perhaps this post will get me points for originality at least.
Check out Dungeon Deities! It's amazing and will make you happy, successful and almost certainly more attractive! It might be true!