WordPress would benefit greatly from supporting MongoDB in terms of scalability and management of images, videos, posts, etc. I think for WordPress.com to adopt a noSQL storage system is actually inevitable, so why not do it now? They've already adopted Nginx as the web server, adopting MongoDB or Hyper table would be the sign of a true leader. To be the first CMS to adopt the noSQL database, WP will have a huge edge as users demand cheaper scalability. The noSQL databases are the future, this is what Google uses, the BigTable storage system, which has an open source version called HyperTable, as they say:
Hypertable is a high performance distributed data storage system designed to support applications requiring maximum performance, scalability, and reliability. Hypertable will be particularly invaluable to any organization that needs to manage rapidly evolving data to support demanding real-time applications. Modeled after Google's well known Bigtable project, Hypertable is designed to manage the storage and processing of information on a large cluster of commodity servers, providing resilience to machine and component failures. Hypertable seeks to set the open source standard for highly available, petabyte scale, database systems.
Read about the anti-database movement.