Why Windows 10?
https://technet.microsoft.com/en-us/library/dn986867(v=vs.85).aspx
https://blogs.windows.com/windowsexperience/2016/01/04/windows-10-now-active-on-over-200-million-devices/
Start Menu
Continuum for tablet / desktop switching
Cortana - your personal assistant
Edge Browser (IE 11 is still installed if you need it for backwards compatibility)
Windows apps on the desktop
Improved built-in apps, e.g.
Mail, Maps, Calendar, Photos, News, Weather, Sports
Office apps now available from the Store
Improved Search capability - in the PC and across the web
Improved Settings app and control panel
Action Center / Notification Center
File Explorer improvements
Windows Store improvements
Phone Companion - Links your phone to the PC and other services.
Task View and multiple desktops - Task switcher
Windows snap to quickly enable multiple side-by-side windows
Automatic updates and continual improvements as Windows as a Service kicks in.
More secure and resilient
Scheduled restarts if a reboot is required
Improved Command Prompt window
DirectX 12 for better performance in games
Support app - find online tips or chat with Microsoft support online (everyone who upgrades to Windows 10 gets a free 15 minute support phone call to Microsoft).
Much easier for developers to port an application from iOS or Android to Windows.
Device Guard, which prevents applications from running unless they are recognised.
Windows Hello, for biometric authentication.
Secure Boot, which prevents malicious software from loading while a computer boots, thereby potentially compromising the operating system as soon as it starts.
MORE DETAILS on Why Windows 10:
• The new Microsoft Edge browser is now your default browser. It helps you search online using the address bar; create a reading list for things you want to read later; view your favorites, reading list, browsing history, and current downloads in the Hub; take notes, write, doodle, and highlight directly on webpages; and enjoy a clean and simple layout with the Reading view. Cortana is also available right in the address bar to help you get things done faster and easier.
○ If a page does not display correctly in the Microsoft Edge browser, you can select "Open with Internet Explorer" in the upper right corner to switch browsers and provide feedback on your experience at the same time.
• Cortana, your personal assistant, is now available on the desktop in the US, China, France, Italy, Germany and Spain. Just like in Windows Phone 8.1, she can display your daily routine, finance, food, music, news, sports, travel, weather, and more.
• Office Mobile (Universal) applications are now available in the Store for Word, PowerPoint, and Excel. These apps are touch-optimized and designed from the ground up to run on Windows 10 on a wide variety of devices.
• Updated Start menu. Similar to the rich menu experience you remember from Windows 7, the updated Start menu enables easy navigation of both desktop and modern applications; the ability to search files, apps, settings, and the web; access your apps, documents, pictures, and computer settings; log off or switch accounts; and more.
• Notification Center. Similar to the notification center that was introduced in Windows Phone 8.1, you now have a persistent list of notifications that you can view and address whenever you like. You can also access links for performing quick actions, such as turning Wi-Fi on or off.
• Task View. The Task View button on the task bar enables quick switching between all your open apps and files, similar to using the keyboard combination of Alt+Tab.
• Windows behavior for modern apps. Modern apps now behave more like traditional apps on the desktop, able to be resized and arranged in "windows" as well as in full screen mode. Move multiple modern apps at your convenience, or scan them all together at once. We also eliminated the Windows 8.1 Charms bar, relocating functionality such as Search, Share, Print, and Settings for modern apps to their respective title bars instead.
• Snap enhancements. Working from multiple apps at once becomes even easier and more intuitive thanks to improvements with Snap. Have up to four apps snapped on the same screen with a new quadrant layout. Snap apps to be different sizes with Smart Fill, and even have Windows suggest what other apps you have open to snap with Snap Assist.
• Network Flyout. Right-click on the network task icon and to access a network flyout instead of going to settings.
• Lock Screen UI. All the user accounts associated with the device are listed in lower left corner and the account cameo picture is now round.
• New shortcut locations within File Explorer. Finding recent files is easier than ever with new shortcuts for "Recent Files" and "Frequent Places" you've accessed.
• Filtered taskbars. Shows only windows on your current desktop by default, rather than across all desktops (global Taskbar).
• Virtual Desktop improvements let you drag a window to a virtual desktop and access multiple desktop experiences – and the apps on them – from the same Windows PC, so you can always access the files and apps you care about.