Gingerbread brings a number of developer features and API changes, most notably, improvements to the Dalvik VM garbage collector and event distributor. Google promises smoother gaming with fewer application pauses as a result of the concurrent garbage collection in Gingerbread, and hopefully some of Android's occasional UI stutters will be fixed as a result - the target is to have sub 3ms pauses while Dalvik garbage collects. There are also updated video drivers which improve OpenGL ES performance for faster 3D graphics, but it's unclear which GPUs will get updated drivers. Gingerbread also brings along a Linux kernel update, from 2.6.32 on Android 2.2.1 to 2.6.35 in Gingerbread 2.3.
Gingerbread adds Khronos OpenSL ES sound APi support, and Khronos EGL support for better OpenGL ES texture, surface, and context support. The other changes for developers include better APIs for native input from sensors and adds support for a few more: gyroscope, rotation vector, linear acceleration, gravity, and even barometer sensors are supported. I'd definitely like to see a smartphone with a barometric sensor, given how inaccurate even WAAS-supported GPS is at reporting altitude properly.
The camera API also has been updated to allow for better support for devices with multiple cameras, most notably devices with a front-facing camera.
Finally Google has added VP8 video compression support and WebM container support, Google's own compression standards. There's also new AAC encoding and AMR Wideband encoding for audio capture.
As far as a changelog, I would not count on one until it's stable for the Droid One.