The Windows 10 upgrade may be free, but it removes Windows Media Center from PCs with the add-on installed. Users with Windows Media Center enjoyed DVD playback without the need for third-party software before Microsoft killed it off. As a result, Windows 10 can't play DVDs out of the box, but Microsoft's $15 Windows DVD Player app puts that feature back.
Fifteen bucks wouldn't be bad if Windows DVD Player was called Windows Blu-ray Player instead, but Blu-ray support is absent here. For comparison, Microsoft offers Blu-ray support on the Xbox One with a free standalone app. Forgive me for the Xbox One comparisons lately, but Microsoft really seems to want to nickel and dime its faithful Windows audience.
The good news is you can play DVDs for free on Windows, as well, thanks to Videolan's VLC and libdvdcss library. Kodi also supports DVD playback without an external module. Both of these media players have long supported local video file playback, streaming from another PC, and extensions to add on additional features all for free. Neither offer native Blu-Ray support, though, and Microsoft is missing out an opportunity to stand apart by leaving Blu-Ray playback out of the Windows app.
Update, August 6 at 1:50 PM: According to Microsoft's FAQ, the company has made Windows DVD Player free for Windows Media Center users "for a limited time."