SOA primarily rests on the principle of "Architecting the system as loosely coupled modules that expose their functionality through well-defined service contracts"
Since it is an Integration/"Wiring" mechanism, scalability as in any other Design is important.

Security is not an add-on in a Design.It is a feature which is of absolute importance and must be thought of at the early phases of Design.
Separation of State and Behaviour is important in a SOA based design.

Important ‘wire’ properties
●Visibility
●Stability
●Security
●Performance

As for Transactions, since it is all about Messaging protocols,the Design focus must be on defining the life-cycle and the co-relations of the Shared states of Objects.
Conversational state on the "wire" (transactions) must be defined clearly in the Message wrappers. (here we assume that the MEP [message exchange pattern is being used])

SOAs are heterogeneous by definition but management and security must span the SOA.
Once you’ve chosen the common application interoperability standards for your organization, the next set of
standards you need to consider involve security and management. A unified security architecture is critical as
you will have many cross-application interactions and if each application has a different security model, it can
lead to security holes and increased costs for managing and maintaining your SOA environment.

For more, you could try to focus on the Design patterns for SOA.IBM's redbook sg246494.pdf could be a good start.

posted October 19, 2007