A Developer's Guide to Windows 10 Preview - Microsoft Virtual Academy
As Windows is moving towards an universal platform, the core of the OS will be the same on computers, tablets and phones. Because of this the core must take as little storage as possible. The differences will become more and more obvious the longer you use the device, what you do on top of that core.
The other one is a language pack for English.
Of course the issue with a build upgrade is that this time we had no advance notification. I believe nobody of us enthusiasts was expecting MS to release the next build so soon after the 10041.
For quite a many the WU starting to download the upgrade came as a surprise so if you were on wireless then I can understand why you just didn't bother to switch to wired.
Thanks to TweetDeck I saw the Joe Belfiore's tweet the moment it was online, wchich was two minutes before midnight my time. Checked the WU, let the PC download the upgrade ESD file, grabbed it and created an ISO and less than an hour and a half after I got the news I had a cleanly installed 10049 up and running.
Actually all my devices (PCs, PS4, etc.) are connected wired. I only use wifi for my portable devices at home.
Downloading 10-49 at the present time. Because of DSL at 3 gb speed it will take some time.