The most important point to realise with Direct3D 10 is that whilst it retains many similarities with previous iterations it was fundamentally redesigned from the ground up. For a start, it's intended to be for Windows Vista only – that is, you will not be running Direct3D 10 applications on Windows XP.
Amongst a number of other technologies, Windows Vista will be introducing the Vista Display Driver Model (VDDM). Graphical features and effects are a much bigger part of Windows Vista than they have been in previous Windows operating systems – as such it requires the GPU to go beyond its current (primary) role as gaming hardware. Take a look at the "Aero Glass" GUI skin for a perfect example of this.
The GPU is to be viewed as a shared resource in the system with multiple applications using and relying upon it – making stability a much more important factor. It's also worth noting that as GPU's become more powerful it is necessary to have a clean and efficient path for utilizing them. VDDM moves much more of the command scheduling and translation into 'user mode' and keeps only the essential parts in 'kernel mode' – such that if the hardware or driver crashes it's possible for the system to effectively restart the driver/hardware and avoid taking the whole system down.
Sharing the GPU is a big part of VDDM; to the extent that the video memory will be virtualized by the operating system. This will in turn allow for resource sharing across threads, which could become an important feature with the recent turn towards multi-programming. Another bonus of the GPU becoming a more central resource to the system is that the "lost device" scenario is gone – so applications no longer need to worry about handling it. However, there is a "device removed" state – which exists for the increasing number of laptops that come with docking stations.
Direct3D 10 also introduces the DirectX Graphics Infrastructure (DXGI) – a common foundation for this new release as well as any subsequent versions (e.g. Direct3D 10.1, 10.2, 11, 12 etc…). Much of the basic low-level resources and operations stay constant and common across most versions of Direct3D such that they've now been isolated from the core runtime. The benefit being that there is a stable and consistent foundation for the API to be based upon, and for application developers it should allow different API's (e.g. D3D10 and D3D11) to share resources.
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