The only hardware-accelerated operations of the GDI are color filling, blitting, alpha-blending, and stretching (with or without alpha). Everything related to drawing shapes (polygons, curves, ...), text, and any UI operation using alternate merging operators (such as XOR, anything but copy and blend), is done in the CPU. It is still partially hardware-accelerated since Windows 7 and up support drawing directly into GPU memory, where Vista had to synchronize the RAM copy with the VRAM copy.
If anything feels snappier in older OSes, it would be because before XP, Microsoft still supported old hardware blitters, such as what was used by DirectDraw. Support for hardware blitting was lost with the switch away from the Win9X architecture and embracing NT. I may be remembering wrong on this, but I believe XP was missing the faster blitting, but didn't support the same level of hardware-acceleration as newer versions of Windows, so in theory, it should have been the least snappy of all...
As to the % of applications using GDI, I'd make it more like >95%, and I'm being conservative. The only application I know for certain uses Direct2D or Direct3D for the interface drawing would be Visual Studio 2010 or newer. And XUL-based applications such as Firefox, where d2d/d3d is supported and available. As far as I know even Java applications which custom-draw the GUI end up doing so with GDI functions.