Benefits of multitenancy:

• economies of scale for the provider through small, experienced IT Ops, a single code base and a single platform
• cost efficiency is partly passed to consumers who pay lower prices.
• I do not agree with Steve that the quality of SaaS is necessarily higher because network-based applications still have major deficits compared to local applications, especially Desktop apps.
• no information silos; instead, improved collaboration and integration

Benefits of metadata-driven architectures:

Metadata-driven architectures are a good choice to implement multitenancy since they provide a polymorphic, dynamic application environment. This allows users of the platform to build custom extensions.

How to build a metadata-driven architecture?

The main idea is to separate a compiled runtime environment (”kernel”) from several (meta-)data layers (data, common metadata and tenant-specific metadata). When a user creates a custom extension, the extension is saved in a metadata directory and created on runtime (thus improving scalability). A potential bottleneck are metadata I/O operations which is why caching of the “virtual applications” + directory search optimzation is a good idea. Force.com provides developers with a WSDL document that lets them generate an API for accessing the Force.com Web services. More information on Force.com-specific development can be found in the very read-worthy whitepaper.