UPDATE 1/21/15 9:30 AM PST - Want Windows 10? You got it: for free. This is a sea change in Microsoft's strategy when it comes to upgrades. Gratis upgrades will be available for Windows 7, 8.1 and even Windows Phone 8.1 users for a full year.
UPDATE 1/21/15 9:47 AM PST - Word, Excel, PowerPoint will be included in W10 for Phones & Small Tablets
UPDATE 1/21/15 9:48 AM PST - The recent document lists roam, so match between devices and Win32 Office and An Outlook mail client for phone and tablet
UPDATE 1/21/15 9:58 AM PST - Microsoft's Cortana voice-assistant is coming to Windows 10 as we expected. Now you'll be able to speak commands and ask questions throughout the operating system.
UPDATE 1/21/15 10:15 AM PST - Every W10 PC and tablet will have the Xbox app
UPDATE 1/21/15 10:20 AM PST - DirectX12 scales tremendously better than DX11
UPDATE 1/21/15 10:22 AM PST - For CPU-bound games, up to 50% performance improvement.
UPDATE 1/21/15 10:28 AM PST - Windows 10 will bring new capabilities (DVR) to existing games and (streaming) to Xbox One games.
UPDATE 1/21/15 10:47 AM PST - "Welcome to a new era of Windows: Windows Holographic" "Want to create a holographic app? Easy. Holograms are Windows Universal Apps. And all Univeral Apps can be made with Windows Holographic."
UPDATE 1/21/15 10:55 AM - "We created HoloStudio to let you create your own holograms and 3D print them"
UPDATE 1/21/15 11:06 AM PST - "Windows 10 is builtfor a world where nearly everything, both at home and at work, is digitally mediated"
LOL - Everyone here is asking, "Where was Windows 9?" Presumably they will address that but we will see.
I will definitely be watching and am curious how they're going to approach the "One OS to Rule Them All" philosophy whereby Windows 10 will run on all device types.
This is what you will see if you go before the Live Blog starts but there is a nifty countdown clock...