We all loved placeholders in OneDrive with Windows 8, you could have your files stored up in OneDrive and you can access them on any of your devices without having to sync up folders beforehand. Windows 10 placeholders were scrapped by Microsoft so you could only sync whole folders across machines.
Demonstrated today at Build Microsoft showed off the new OneDrive file syncing system coming to Window 10 called OneDrive Files On-Demand. This syncs up folders and files using placeholders, so a file stored in OneDrive can appear to be on the file system and then OneDrive brings down the file when it is accessed. The system is part of the OS and not just a shell feature and seems more advanced than the Windows 8 version.
Also announced is Windows TimeLine which enables you to go back to a point in time with apps and it using Cortana it enables you to continue working where you left off. When you switch from one device to the next Cortana will prompt you to continue working on the app, it could be Powerpoint or even a news reader app. It makes moving from device to device very simple even if you are going cross platform.
Another create feature is a Cloud Clipboard so you can copy and paste between devices including iOS, Windows and Android. This is a great tool and will mean the end of emailing yourself pictures and text.