amber is nothing just some tweaks done by nokia and released new lens called smart cam. The main focus should be on general release distribution 2 made by Microsoft.(gdr2).all the performance, battery improvement, FM radio and bug fixes in OS is handled by Microsoft. nokia added flip to silence which was earlier on my lumia 900 now ported back to windows phone 8 series by nokia(520-1020),adjust screen colour profile is a move by nokia as this feature is hardly seen on smartphones these days. Glance , double tap was first seen on nokia n8(Symbian) and now ported to windows 8
,call blocking is also a nice move as it was the most requested feature in lumia ( Samsung ativ series have this on windows phone devices before the gdr2). htc seeded the gdr2 update(no software was released or tweaks by htc
just gdr2 update by Microsoft) earlier which created bugs and issues in phones like hearing music for longer time bricked the phone ( the audio chip related problem). htc later resolved this issue by seeding another update to the htc windows phone series. there are still some bugs in amber update seeded by nokia (images taken by camera on l920 shows different colour(blue shows green,pro cam bugs).hope I made some the clarification discussed here.glance screen is a huge battery sucker on l920(due to non amoled display as it has an IPS display).