According to Microsoft's lifecycle calendar, Windows 7 left mainstream support on the 13th of January and has entered “Extended Support”. This means that the operating system will still receive security updates, but not non-security ones, and “requests to change product design and features” will not be accepted. While the OS is over five years old, it is still very popular, especially among PC gamers.
My concern is that this occurred while we anticipate major changes to the Windows platform. While I never really expected that Microsoft would release DirectX 12 for Windows, there was still hope that we would see a pre-release or developer build while Windows 7 was still in mainstream support (despite being several driver models behind). Now that the window has closed, so to speak, that hope is diminishing. Windows 8.1, on the other hand, might be okay, but I have no idea why you would want to stick with it over Windows 10, especially if it is a free/cheap update.
Besides DirectX 12, I am also concerned about Microsoft cutting off first-party web browsers at IE11. Sure, it is a much better place to end than IE8 on Windows XP, and the end-user could always install a third-party browser, but it could lead to problems for web developers. It is much easier to say “keep Internet Explorer up to date” (heck, even Microsoft is saying it) than the alternative, “use a different browser”. There are still many features under consideration (Shadow DOM being the most interesting for me) that would be nice to have, and not need to worry about the fraction of a fraction.
But at least it will be kept secure until 2020.