In general, I've found that configuration of Direct3D and its descendants is a chore due to the verbosity of the APIs. While I don't have much experience with GDI, I'm fairly sure that it just acts as an extension of WinAPI and would be much easier to configure. However, at the same time I'm fairly sure that GDI only allows for software rendering, which would cause a nice performance hit for more intense 2D applications.
The Direct2D API is designed to interoperate well with GDI, GDI+, and Direct3D.
So it sound like you'd typically use GDI for the basic stuff (as it's easier/more cost effective) and Direct2D when you need the power
I've mostly used GDI, which I see as just a regular part of the core Win32 API  (well, it's part of Petzold's Programming Windows book ). I've also used GDI+, which is pretty easier to pick up once you know GDI. And some DirectX (inc. Direct3D) which is rather more involved!
 The core of the Win32 API is provided kernel32.dll (base services), user32.dll (UI services), and gdi32.dll (graphics services). I'd also add advapi32.dll (advanced services) as it provides registry access, etc. And then you have the shell, the common controls and dialogs, network services, etc, etc, etc.
 Programming Windows®, Fifth Edition
Table of Contents http://www.microsoft.com/mspress/books/toc/2344.aspx
Disch, SDL 2 isn't officially out. The wiki has been defaulted to SDL 2 though for nearly 3 years now. It's ridiculous. Their was a recent push to get SDL 2 out the door (thanks to Ryan Gordon) because of development on the Humble Indie Bundle games. Unfortunately, it's *still* not out and developers in IRC say the same thing they've said for years, "It'll be out when its ready". I've pretty much given up hope to be honest.