TheBrain, as a commercial AppExchange product on the Force.com (Salesforce) platform, in my opinion would fill a huge existing gap: The ability to easily visualize either system level metadata (object relationships) or even more importantly hierarchical record-level data relationships across those objects.

The applications of TheBrain from this perspective seem endless. I've listed a few below:

Administrative/Developer/Architect
- Implementation Modeling of initial Data Model Architecture
- Actual Data Model (objects, fields, etc) metadata importing/exporting into Sandbox
- Ongoing Production level Data Model evaluation for support and trouble shooting purposes.

End User
- Data Visualization for improved contextualization of relationships
- "Salesforce Knowledge" visualization based on content tagging and libraries
- Record-Level visualization of related hierarchical parent or child relationships
- Team collaboration tool for projects, process management, external system document storage, etc...

I could go on, but for the sake of time I won't but to say it would prove a worthy exercise, if it hasn't already been considered, to evaluate the potential revenue and marketing channels the Salesforce AppExchange could provide given the lack of UI / Data Visualization flexibility of the platform.