Edit: It is all about InnoDB engine, which is now developed and fixed by Oracle. Others, notably Percona, have its own set of patches which adds some features to what Oracle releases, while MariaDB is mostly re-branding.
They are also working on a new table discovery engine, which will bring Transactional DDL to MariaDB, this to me makes MariaDB far more superior to MySQL. They are innovating, and they are open about it. https://mariadb.atlassian.net/browse/MDEV-3808
Oracle have only innovated on the niggles of Mysql, like the unencrypted ~/.my.cnf files to appease their "Enterprise" customers.