The entire SoC line of Atoms is based around using the PowerVR core, which has never supported full OpenGL. Not to mention that OpenGL 4 will be compatible with OpenGL ES, so it's a moot issue.
Having Intel as a supplier and a well known architecture.
If the power efficiency gains in Medfield are accurate, ARM may be given stiff competition. And MeeGo is a "real" OS. It's not Windows, but it's still a real OS.
You can't exactly load XP on a Medfield system and have it just work, many of these systems are dropping legacy functionality that older, closed OSes don't support at all. You can still use Wine, though