In OpenGL all the fun stuff (e.g. the stuff that's in DX9+) is in vendor specific extensions.
A bit like vendor prefixes in CSS, except they can actually be implemented differently, have different names, etc. It's BAD news, and the main reason why 99% of AAA games use DirectX on Windows.
There are few things one might miss in core OpenGL 4.2. If anything any features exposed in extensions building on OGL 4.2 are so new or experimental that D3D doesn't have them at all.
You'll get hundreds of bug reports about game crashing on start.
Testing on old hardware (for <50 000 triangle games it's Intel GMA and Geforce 4 MX and some old Radeon) is a good way to be sure it works on most of computers. Of course game needs to support those old cards only on lowest quality settings.
In OpenGL all the fun stuff (e.g. the stuff that's in DX9+) is in vendor specific extensions.
A bit like vendor prefixes in CSS, except they can actually be implemented differently, have different names, etc. It's BAD news, and the main reason why 99% of AAA games use DirectX on Windows.