It is too early to talk about the backward compatibility of apps on Windows 10. According to Microsoft, this is a new Windows based on the "mobile first, cloud first" strategy. While the company has tried to avoid radical changes in the UI, there are huge changes behind the scenes. According to the official announcement, the new Windows 10 is built from "ground-up". In a few places, they have mentioned about backward compatibility as well.
Here is what Microsoft says about backward compatibility:
"We know that app compatibility is critical for business. While we strive to have great compatibility with all existing apps, we've added tools for compatibility testing.".
So, I think we should be prepared to see a lot of compatibility issues and non-compatible apps. Windows 8 introduced HTML 5 as the default language for building apps for Windows. It is unclear whether that will remain the same or not!