It depends on your definition of "critical," but yes, Windows 10 has dropped some features on which some people rely. The one most people seem to be upset about is Windows Media Center (Do not confuse this with Windows Media Player, which is still installed).

Media Center is a Windows program that lets you watch and record TV using third-party TV tuner cards in your computer. It is also a nice way to manage and interact with your videos, music and other media.

Listed below are other features Windows 10 dropped:
  • If you have Windows 7 Home Premium, Windows 7 Professional, Windows 7 Ultimate, Windows 8 Pro with Media Center, or Windows 8.1 Pro with Media Center and you install Windows 10, Windows Media Center will be removed.
  • Watching DVDs requires separate playback software.
  • Windows 7 desktop gadgets will be removed as part of installing Windows 10.
  • Windows 10 Home users will have updates from Windows Update automatically available. Windows 10 Pro and Windows 10 Enterprise users will have the ability to defer updates.
  • Solitaire, Minesweeper, and Hearts games that come pre-installed on Windows 7 will be removed as part of installing the Windows 10 upgrade. Microsoft has released our version of Solitaire and Minesweeper called the “Microsoft Solitaire Collection” and “Microsoft Minesweeper.”
  • If you have a USB floppy drive, you will need to download the latest driver from Windows Update or from the manufacturer's website.
  • If you have Windows Live Essentials installed on your system, the OneDrive application is removed and replaced with the inbox version of OneDrive.