Weird, i still use DX9 where possible because it looks the same as 11 to me and usually reduces resource usage by a lot.
I'm lucky sometimes to have the option between 11 and 9, but i doubt DX12 games would continue to have that choice... unless of course people continue to use Windows 7.

I wonder if DX12 caters towards optimized scalability, or just system hogging technologies like 11 did.

DirectX 12 is supposed to do for every game that uses DirectX 12 as ATI did with their Mantle technology. You've heard the phrase "code to the metal" tossed around a bit, especially when it comes to Mantle or even the coding for the new consoles. It's basically just a way of removing a lot of the API layers between the video card and what the game/program is telling the video card to do.

It should, at least in the case of Mantle, increase performance on lower end machines (namely those with a lower end CPU but a decent GPU) as it would allow developers to push more complex issues to the powerful GPU instead of the CPU.

DirectX 11 is also better performing compared to DirectX 9 if all things stayed the same. If developers did not add in any additional features and kept all other variables the same, DX 11 should be a better performer in a scene than DX 9.

The problem is, developers took this extra headroom on performance and decided to add in new visual features to their games in DirectX 11 mode that quickly closed the performance gap. They add in things like tesselation (example from Nvidia), TressFX, Percentage-Closer Soft Shadows (example from Nvidia) (PDF link) that is in AC Unity now and has a HUGE hit on performance if enabled, all matter of new AA modes that also have tremendous impacts on the visual fidelity of a scene at the cost of a lot of performance. And so on. Though admittedly I don't believe some of these are limited to DX11, they are merely only found in DX11 versions of a game because developers think it has more performance headroom than it really does compared to DX9.

So there are certainly visual differences between DX9 and DX11, they just aren't always super apparent. There will probably be some between DX11 and DX12 though its major focus will be on performance gains.

Here's hoping.
I've tested out DX11 options in various games but never found the improvements to be worth it, the reduction in performance always left a bad impression on me, like the performance loss wasn't earned.

Update: Microsoft clarified that what Richard Huddy said may not be true, but doesn't give their own response, so it's probably still true.