Mark Smith, a Developer Advocate for MongoDB, discusses the database's underutilized features that are particularly beneficial for developers. While MongoDB is a versatile general-purpose database, Smith highlights three lesser-known features: TTL Indexes, Capped Collections, and Change Streams, which can enhance data management and application functionality. TTL Indexes allow automatic removal of documents after a specified time, making MongoDB an effective platform for data applications rather than just a database. Capped Collections provide an efficient way to maintain a ring buffer of documents by setting a maximum size or number of documents, which are stored in insertion order and automatically purged when limits are reached. Change Streams offer a real-time stream of database changes, enabling developers to implement database triggers in their preferred programming language. These features, Smith argues, can significantly improve efficiency and functionality in projects using MongoDB, suggesting that developers can optimize their use of the database by incorporating these tools.