I can't +1 this one enough. I have some developers on my team who insist on using Docker on a Windows setup, and it is painful to use with VirtualBox.
OSX is better because it doesn't feel too different from Linux (aside from setting docker machine ENV variables). Still virtualized so you take a performance hit.
The Docker beta that was announced is using Windows 10 builtin virtualization APIs. In theory when utilizing kernel virtualization features there will be less overhead (closer to the metal), but still overhead.
I wonder how they will make Ubuntu happen on Windows. Reading some of the comments, some speculate a subsystem, while others suggest an interoperable interface.
Edit: reading bitcrazed's comments it looks like it will be implemented a la WINE. No need to recompile binaries made for Linux x86; you'll be able to run apt packages from Ubuntu out of the box.