The details differ, but each Platform as a Service vendor provides roughly the following:
- Web based development tools: graphical environment for building business productivity apps and deploying them immediately to an on-demand server.
- Proprietary server technology: the applications and data are locked into a unique SaaS environment. Goodbye any hope of managing IT standards consistently!
Why would any enterprise want another company to have semi-exclusive control of their business logic and data?
To be sure, the SalesForce platform may well be an excellent way to extend existing SalesForce applications, as ZDnet points out here. It does not make sense, however as a stand-alone platform, particularly in comparison to more accepted platforms like J2EE and .NET.
At ActiveGrid, we believe that the whole distinction between on-site and on-demand deployment is an artifact of poor tool design. The next generation of web development tools will make it transparent whether you are deploying to your desktop, to the data center or to a SaaS hosting environment like Amazon's Electric Compute Cloud. Programmable web has a nice summary of EC2 applications here.
The next generation of virtualization will occur when enterprise developers can make on-the-fly decisions about whether to host applications on-site or on-demand. Put succinctly, SaaS manageability goodness is not enough to overcome proprietary badness.