I've worked on a commercial OpenGL game for several years and most of my work was in the graphics part of the code. Speaking from experience, most of the problems we ran into was due to people not having up-to-date OpenGL drivers installed.
Most people (not most hard-core games, but most casual and non-gamers) have integrated graphics solutions (integrated Intel or mobile AMD/NVidia in a laptop) and rarely or never update their drivers from when they first get their machine. It works well enough for them to surf the web, e-mail, do their work (editing Word/Excel/PowerPoint docs) that they never have an urgent need to update their video drivers. Also, many of them feel that updating drivers is a difficult thing to do (too technical for them) and are afraid that they will mess up their system.
In addition, the OpenGL support of integrated video chipsets is not necessarily the best to begin with. And Intel/AMD/NVidia do not provide updates for their older integrated video chipsets which are still in use by many people. So, some of these people were stuck with older drivers with known bugs in the OpenGL drivers.
In reality, there are a lot more games that use DirectX than use OpenGL (easily 10 to 1 ratio). So, Intel/AMD/NVidia have not had too much incentive to keep the quality of their OpenGL drivers on par with the quality of their DirectX driver. But, the quality of the OpenGL drivers in the past few years has greatly improved.
So, the good news is that the quality of OpenGL drivers is improving. The bad news is that a lot of people are still using (or stuck with) older, buggy OpenGL drivers.