Difficulty of porting to WP7 has little to do with the lack of OpenGL ES, particularly for a game like Angry Birds which in terms of hardware dependence basical

Difficulty of porting to WP7 has little to do with the lack of OpenGL ES, particularly for a game like Angry Birds which in terms of hardware dependence basically just renders sprites. Assuming similar hardware capabilities and a sensible abstraction layer this part of porting is surprisingly straight forward. The big issue is that all WP7 development must be in managed code. Unlike the PC You cannot mix and match managed/native code and libraries. This is a big problem for companies with existing games and tech built around C/C++, and a huge problem for smaller companies who have built their games on free/OSS libraries (Bullet for example) and do not have the resources/skillset to develop managed alternatives. So you are right that porting to WP7 requires a rewrite, but it's entirely due to the requirement of managed code and not because of OGL ES v XNA.