I'm installing build 9901 as I write this, but this newly leaked build of Windows 10 provides a glimpse at a lot of changes, including a full Cortana implementation, several new apps, some new UI treatments and an evolving new Store experience. For anyone still struggling with build 9879, this new build is a reminder that things are about to get a lot better.
Setup. Unchanged from previous builds, but I don't think I ever called out the new look and feel of the out of box experience (OOBE), which actually debuted in build 9879. I assume this is designed to make the experience more consistent with Windows Phone. But it's a bit shockingly bright on a big screen.
New wallpapers. Not a big deal, but I know how much some people like the superficial stuff, and eye candy is always nice. Plus, the taskbar and title bars are black in this build.
Cortana. Microsoft's digital personal assistant isn't just present, it's available in vastly nicer form than we've seen in leaked builds, with her own taskbar-based search bar.
New title bar treatment in Modern apps. The title bar hasn't changed on traditional desktop windows in this build, but it has in Modern apps, with a cleaner new look for the minimize, maximize/restore, and close buttons and the removal of the window button (on the far left).
Settings pane now appears in Modern app window frame. Through build 9879, the Settings panes in Modern apps worked as they did in Windows 8.x, displaying attached to the right edge of the screen. Now, they display within the actual app window, which makes much more sense.
Store Beta. While Microsoft has tried to make a Store app that scales to the screen and the sheer number of apps it now contains, previous efforts have always fallen short. The new version appearing in beta form in this build is incomplete, but it now uses document-style vertical navigation (instead of a panorama) and puts apps, games, music, and movies & TV all in one place. Just like Windows Phone.
PC Settings replaced by Settings. A new Settings app shows how Microsoft may replace both PC Settings and Control Panel with a single UI. I mentioned previously that Microsoft will never actually be able to get rid of Control Panel, but it could certainly integrate that functionality—including support for third-party control panels, which is the big issue—into a new interface. Maybe this is it.
Xbox app. In Windows 8.x, we have a confusing array of Xbox apps, including Xbox Music, Xbox Video, Xbox Games (never even updated past the Windows 8.0 look and feel) and Xbox SmartGlass (in separate version for Xbox 360 and Xbox One). In build 9900, we see a new Xbox app, but it appears to be a new Xbox SmartGlass.
Other new apps. The Camera app has been replaced by a new Camera (Beta) app. There's a new Remind Me app that can run in the background. Contact Support and Get Started are new, but minor.
Updated apps. Alarms, Calculator and Maps have been updated. Most notable, perhaps, is a major update to Photos, which now resembles the Xbox Music app. (You may recall that this app was severely downgraded in Windows 8.1.)
I've almost got the build completely installed in a VM, so I'll report back soon with more information: A real hands-on is always better than relying on others.