Wuala has many, perhaps all of your desired properties. The two killer features for me are client side encryption and selective syncing, you can separate defin

Wuala has many, perhaps all of your desired properties. The two killer features for me are client side encryption and selective syncing, you can separate define for each client which folders are synced. Wuala even supports streaming of files. With their virtual file system you can start reading not completly downloaded files. The disadvantage of all of this is the increased complexity in the user interface. That's the beauty of Dropbox, the simplicity. But now, after getting used to Wuala, I wouldn't want to miss it.