All mainstream cards do support OpenGL - Even most non-mainstream (like Matrox cards) as well as high-end professional cards (like FireGL, Wildcat and Quadro).
While this is true, a large percentage of PCs don't come with OpenGL drivers installed. This applies to all those PCs that come with Intel's motherboard graphics chipset, which is what all the low-end Dell's come with. You have to go to Intel's website to get the OpenGL driver, which is definitely something you don't want customers to have to do.
On Linux and Apple it is no problem as they focus on OpenGL.
Only Windows is a problem
Only Windows is a problem
Exactly. And Windows is 90+% of the market, unfortunately. I fully support Mark's decision to go with OpenGL, though, because I understand what a pain it would be to support both options. But it would be nice to have alternate versions of the 2D modules that use DirectX. I expect we'll see that as a third party add-on.