The most significant changes are full support for ID3v2.4 and optimization of MP4 files - along with many, many bugfixes, changes and other new features.
I've replaced the version on the server with Mp3tag v2.36a, which defaults to ID3v2.3 with UTF-16 instead of ID3v2.4 with UTF-8, due to many support questions regarding applications with missing or buggy ID3v2.4 support.