I've recently opened a discussion on the Orchard boards about graph databases. A proper graph database under Associativy could greatly enhance its performance and scalability. Associativy now actually stores the graph in an SQL table, however for quick traversal it also loads it into memory. This is fine, but most likely won't work consistently in a multi-node environment like a web farm and can lead to performance problems with very large graphs.
Share your ideas in the discussion! Not only Associativy would need a graph database.