It doesn't sound like you've developed much in the way of commercial quality graphics-centric software for "middle tier" end-users.
Long story short is that if you ship a 3D software title that's designed and marketed to be run on as many machines as possible, modern OpenGL drivers tend to greatly lag or be unavailable on many Windows machines. Vista and especially Vista 64 really made this worse since maintaining and upgrading GL drivers for middle tier GPU hardware is generally at the bottom of the priority list for 3D driver development. Meanwhile, D3D support tends to be solid, speedy, and well maintained considering especially when you consider what hardware they have to work with.
I'm a senior engineer at a software company that manages a cross-platform codebase that puts OGL and D3D under the same roof (we ship products for OS X and Windows). One of our older products requires OpenGL on Windows due to not wanting to rewrite all its shaders for D3D, and the number of support tickets that we get from that TOWERS our the number of support tickets from our other D3D titles combined. It's downright pathetic how poor OpenGL hardware and driver support is on these middle tier Windows machines.
The last thing the guys and Intel and Nvidia that maintain the drivers for built-in chips that go on the typical Dell machine give a crap about are the state of their OpenGL support. I'm typically surprised to see them have support past 1.3.