D3D11 works on Vista as well, as long as they have the proper platform update. It's actually possible to query for the required update and prompt a download/install from Windows Update, at least if you're proficient at making installers.

Purely in terms of being a graphics API, I would say D3D11 is unquestionably better than D3D9. It's cleaner, leaner, and let's make use of newer hardware capabilities. Will that actually matter for your game? It depends on what kinds of game you're making, but in general the graphics API isn't going to become a limiting factor until you start having a really sophisticated level of graphics tech. If you want bleeding-edge graphics, then yes you want D3D11. If you want sprites or rudimentary 3D, the API isn't going to be that important.

Either way I would recommend checking out the latest Steam and Unity hardware surveys so that you can make an informed decision regarding the baseline hardware and OS that your game will support.

That dilemma is what keeps developers(like me) from actually creating the engine. When I would have finished writing the engine in DirectX11, DirectX12 will be the cutting edge. So unless Windows 8 and Xbox One doesn't support DirectX9(which it does), I perceive no reason why I should aim at the latest instalment of DirectX, taking into consideration also that my graphic demands are minimal(2D texture billboard-heavy).

I am talking about building an indie game from it, not an engine competing with Unity or UDK.

Since Direct X 11.2 will be Windows 8.1 and Xbox One exclusive, and only Windows 7 and 8 benefit from Direct X 11, is there a significant possibility that Microsoft is doing the same mistake that happened with Vista(most game developers sticked with Direct X 9)?

Is it really worth the effort to develop with Direct X 11 as an indie developer?

Depends on your game and target market. I would realistically limit your options to either D3D11 or OpenGL; D3D9 is a dead, legacy API for a legacy platform.

In terms of learning graphics, I recommend starting with D3D11. It more closely matches real hardware and will all but force you do "do things right" unlike OpenGL which still results in goofy questions around the Web debating the merit of vertex arrays vs vertex buffers or performance concerns with glBegin or whatnot.

If you are targeting parts of the world outside of NA and western EU, you'll find that pirated XP is still the most common OS (though it's shifting towards pirated Win7). If you are targeting Mac and Linux you're forced to use GL and not D3D. If you research the target markets for your game you may find that supporting China or Mac users may be important to your bottom line. GL is the better choice for portability as it is supported by XP, OSX, Linux, and most mobile platforms.

Assuming you are sticking with NA and EU as your target markets and don't think you need non-Windows support, however, stats do seem to indicate that D3D11 has sufficient penetration in the market and is safe to depend on with relatively minor lost sales opportunity.

Even if you do think you need the portability you can make a strong argument for building your game with D3D11 initially and porting to GL afterwards. D3D11 is easier to work with in modern usage (old-style GL is clearly easiest, but it's deprecated for very good reasons), D3D drivers tend to be less buggy, and Microsoft's dev tools are significantly better than anything I've ever seen for GL, making it much easier to get your game up and running with D3D11 than it is in GL especially if you're doing anything even remotely advanced.

The key point to take away from all this is to research your target market. My opinions on APIs or research done for my own products are not necessarily relevant to you and you should not bet the success of your game on feedback from strangers on the Internet.

I have read an interesting blog post from a respectable German developer, where he specifies that DirectX11 all together is not so relevant for reaching huge audiences(the emerging Chinese, Indian and middle-eastern).

Also, Microsoft itself specifies that DirectX9 is the version to develop with for maximum compatibility.

Purely in terms of being a graphics API, I would say D3D11 is unquestionably better than D3D9. It's cleaner, leaner, and let's make use of newer hardware capabilities. Will that actually matter for your game? It depends on what kinds of game you're making, but in general the graphics API isn't going to become a limiting factor until you start having a really sophisticated level of graphics tech. If you want bleeding-edge graphics, then yes you want D3D11. If you want sprites or rudimentary 3D, the API isn't going to be that important.

Hi, may I ask which hardware capabilities supported by D3D11 (besides Tesselation) are the most important for modern 3D games in your opinion? Or, put it another way, what supported hardware features does a modern 3D game benefit most from when using D3D11 compared to D3D9?

Compute shaders is the big one. Combines with the ability for arbitrary read access from buffers and textures, it opens to door to all kinds of new techniques. We haven't even really scratched the surface of what's possible with compute. Other notable mentions:

• Access to MSAA data
• Access to depth buffers (possible in D3D9 through driver hacks)
• Better instancing support (you can access arbitrary buffers now in vertex shaders, which makes instancing much more powerful)
• Constant buffers (reduces CPU overhead)
• Up to 128 textures
• Decoupled textures and sampler states
• Integer math in shaders

How many devices and platforms I can target, if I invest time in learning OpenGL-like APIs. PlayStation 3/PS4, WebGL, Android, iOS, Linux, Mac and of course all versions of Windows ?

Personally I think that if your resources and skillset are such that it would take a prohibitively long time to implement a DX11 engine, then you should probably consider using an existing engine. An engine like Unity has many engineers for using the latest API's, and can abstract the features in such a way that you don't have to bear the entire burden of targeting multiple platforms with varying feature sets.

If you are going to make an engine, then I would say at some point you've just got to saw "screw it" and start making the game using whatever technology and API's you're comfortable with. Like I said early for the majority of games D3D9 vs D3D11 vs GL really isn't going to affect the actual game and what you can do with it, it'll just be an implementation detail. If it turns out your game is awesome and you want to target new platforms, then you can always port it after and change the graphics API if needed.