I am very excited about the upcoming DirectX 12. I have always been standing on the side of PC, Windows and DirectX. Currently I code in DirectX 11 on Windows 7 at home. Upcoming Windows 10 with free upgrade from version 7 and 8 (and the Start menu back on its place) looks like a good system. Together with it, a new version of DirectX will be released. Let us summarize general information about this new graphics API that are publicly available at the moment.
DirectX 12 will require new Windows 10 to run. Some may complain that it is a political decision to force users to upgrade, just like DirectX 10 required on Windows Vista and didn't work on XP, but in fact it is related to introduction of new WDDM (Windows Display Driver Model) 2.0, just like Windows Vista introduced first WDDM.
It will not necessarily require a new GPU. Graphics hardware vendors announced that DX12 will work on many devices that are already on the market (both discrete and integrated graphics cards), just after upgrading the operating system (and graphics driver or course).
The API will change significantly. Maybe it's not a completely new API, but it's not a small evolutionary step either. There will be many new concepts to understand and new types of objects to learn how to use. This is what I like about technologies managed by just one company (like C#, DirectX) as opposed to these managed by a committee (like C++, OpenGL) - they have clear direction, progress quickly and don't hesitate to make bold moves.
It will be more low-level (like graphics API-s on game consoles). In the way similar to AMD Mantle, Apple Metal and Vulkan (previously known as glNext), it follows a new trend of offering to PC graphics programmers a more fine-grained control over contemporary GPU hardware, with thinner driver and more responsibility on developer's side. It can result in less CPU overhead, more parallelizable code and finally more draw calls, more FPS etc. - basically better graphics.
Here are some interesting links:
First and foremost: Direct3D 12 Graphics @ msdn.microsoft.com. Microsoft says "these information relate to pre-released product and may be substantially modified before it's commercially released", but you can already find there all Direct3D 12 Programming Guide and Reference, so basically the whole API is already public and you can start learning it.