The version notes for the beta include what's new in the Windows 10 version of the app:
New: We are previewing a brand new look for Wunderlist. What does that mean for you? Well, on Wunderlist for Windows 10 your to-dos will be easier to read and, our backgrounds are now crystal clear, so you can fully appreciate the cute puppy background.
New: Look to the top left of the app and find the three lines. You can now collapse the sidebar with a simple tap on this icon, giving you more room for your to-dos. Want the sidebar back? Just tap on the icon again.
New: We've also improved accessibility with better keyboard support and high contrast themes.
While the app is available for all Windows 10 users, it's still considered a beta, so there will likely be a few bugs. People who want to provide feedback back to the Wunderlist team about the app can do so by emailing firstname.lastname@example.org.