"want to play this game? too bad you've got an nvidia card"
Most game engines already need to have multiple specialized implementations (almost like 1 per platform). Now we almost only need to implement at least 2. There's still D3D11, the OpenGL family and all its whack. Pick one of those to fallback for Nvidia cards. Nvidia cards haven't proven to do much outside of the Windows platform, so here's a hint: Pick D3d11.
Mantle = 1 damn good API for PS4, Xbox One and AMD-graphics based PCs.
That's a lot of simplification for the programmers job, not to mention it sounds like a damn good API.
why aren't they getting involved with opengl, it already is an open platform that requires some more love from the vendors.
Because Khronos Group is a load of bs and AMD is acknowledging that fact. I can see why they've put low priority on OpenGL, greater on D3d11 and even greater on Mantle. Good for them. AMD gets to show how it's done.
Also, lookup Carmack's speculation regarding Steambox and Linux if you lack any imagination to wonder how AMD isn't inclined to cater to Microsoft, who already have turned D3D11 solely into a monopolization asset with no love for the rest of us. D3D11.2 bound to Microsoft's latest and greatest s*** on the world? I think AMD would like to say "No thank you." Portability boost.
Nvidia has no option but to dedicate more resources towards kicking Khronos Group in the rear to reshape OpenGL into an API that can compete. That's their best bet. Since AMD already dominates the console market (they control the most pervasive platform), I think it's unlikely Nvidia will try to create their own standard for only Nvidia-graphics PCs. That's a stark difference to the entire platform of PS4 + Xbox One + all AMD-graphics PCs. And the people who buy Nvidia cards are stupid anyway. As a fact, just read any legit benchmark comparison: AMD makes the most powerful cards, not to mention the cheapest