No official list exists. However, they publish the build scripts they use here: https://code.launchpad.net/~ubuntu-on-ec2/vmbuilder/automated-ec2-builds.
You can check them out with the following command:
bzr branch lp:~ubuntu-on-ec2/vmbuilder/automated-ec2-builds
Most of the changes occur from packages installed. For example, the meta package used is ubuntu-cloud instead of ubuntu-server. This includes a different kernel, cloud-init, aws tools, and probably other packages. Cloud-init itself does quite a bit such as changes to apt-get, downloading ssh keys, and loading scripts from amazon user-data. I don't know for certain, but I believe it also makes the ubuntu user and changes your sshd_config to disable PasswordAuthentication. You would need to check the cloud-init deb for that.
Bottom line is that you are unlikely to find all the changes. However, you seem to have found the most important ones. It is the changes to login that really matter. Everything else is just the addition of tools and packages not found elsewhere. If you want to truly understand the AMI and how it works with amazon, I highly recommend learning cloud-init. Cloud-init is a powerful tool for doing some very advanced things with AMIs.