There is one thing you need to be aware of to get an idea why this is a very big win for AMD.

While AMD is a slightly larger company than Nvidia, Nvidia are much more focused in their production since AMD competes in more markets, markedly the CPU segment, where they are competing against Intel, a much, much larger company (10 times more revenue).

This means that in the graphic card segment AMD has less resource available than Nvidia, and have a hard time giving the same amount of external support for game developers as well as individual driver optimization. In this, DirectX 11 clearly supports the company with most available resource (Nvidia), since it requires vast amounts of resource to optimize individual games.

DirectX 12 on the other hand gives this control back to the developer, giving them more control over the allocation of resource. This in turn means that hardware will be a stronger driving force of computer graphics in the future. Since AMD have a weak point in their ability to produce individual drivers and spare manpower for direct support of game developers (due to lower resource available), the outcome of directX 12, which empowers developers and in part removes the need for individual driver optimization, works strongly in their favor.

In any event this is good for the community since it progresses development and competition! :)