Most SaaS platforms intend to provide the following:
• Tenancy – The ability to distinguish one user from another in the data and execution aspects of a hosted application is a major tenet of SaaS. Generally, the concept of tenancy is void in traditional on-premise installs and can complicate architectures beyond what was traditionally accepted.
• Scalability – The idea that a successful application will buckle under its own popularity is never good. Being able to accommodate your aggregated customer base is a must, and planning for success is a requirement.
• Reliability – What good is a SaaS application that isn’t up?
• Hardware Infrastructure – As a vendor, one of the operational headaches of SaaS applications is dealing with an enterprise-grade hardware infrastructure.
• Value Added Services – A good platform should endow the application it hosts with value beyond what was developed by the vendor. The value should benefit the end user.
• Ecosystem – As the number of vendors that host their applications on a given platform increases, and as the number of users using those applications increases, an ecosystem begins to develop. Ideally, this ecosystem allows all parties the ability to investigate and exercise their right to connections between ecosystem members, deriving value beyond that offered by any single SaaS offering.
Here is a good white paper on Saving Money with SaaS