The main difficulty in porting has less to do with OpenGL itself and more to do with the number of platform-specific issues you encounter when you're trying to port code that is highly optimized for specific platforms.
Or maybe you decided to write the whole thing using multi-platform libraries like Qt in the first place, so that you don't have to change hardly anything. Or maybe you wrote it in Visual Studio using .NET heavily and you have to practically rewrite everything from scratch. It's totally dependent on the decisions you made when writing the original program.
So when you change platforms from, say, the Xbox 360 to the PS3, those assumptions you made about the OS, the CPU, the chipset, and other minute details are suddenly completely, horribly wrong. So the guy who wrote all of that highly optimized code has to write it again to optimize against a completely different set of quirks.