Virtual Desktops is one of the largest interface changes for Windows 10. It is an organizational feature, which allows users to group relevant windows together. This puts them out of sight when you don't want them but also easily accessible when you do, and with the programs that you would want to use alongside them.
Microsoft is wondering about one specific area though: should running applications be visible in other desktops? Some believe that a virtual desktop should only show icons for applications in the current context, and alt+tab should behave the same. It makes sense, because otherwise you're exposing the clutter in the taskbar and alt+tab, which are some of the most visible places. Others want everything to be visible at all times, or alt+tab-able to at all time. I guess this is to reduce the clutter on the desktop, without touching the clutter otherwise?
Microsoft has stated clearly that both cases will be user-selectable because there is enough usage on both sides. A user can have virtual desktops affect the taskbar or not, or alt-tab or not, independently. The question they are trying to solve right now is “What should the default be?”
If you are a Windows 10 Technical Preview user, and you care about the adoption of Virtual Desktops, Microsoft might push you into one or the other camp. Later, they will pop up a notification to ask your opinion once they feel you have used it long enough.
This is one area that your vote will influence Windows 10 in a very specific way.