Guiding Principles, important statements to make when thinking about SOA in general.
Well, keep it small and simple (K.I.S.S.) I guess... Try, when starting with SOA adoption define a meaningful boundary for the SOA adoption. Start small, start simple. Not only with the SOA implementation, but also when defining the scope. Try a proof of concept; try experiencing implementation of a few (sharable) services in production. Get your experience with them, and try to see how you can learn. Only then you should be ready to grow from there, in what is probably a multi-year effort, towards the intended initial scope. The scope itself - how far you adopt SOA - can be increased incrementally in a programme approach. In SOA, the big-bang approach is recipe for failure. It may be better to start off small and iteratively see how you can 'grow your garden' towards the 'end state'. But please remember, a garden is never finished and constantly needs maintenance and attention. In a garden usually there is also some waste; sometimes a bit more and sometimes a bit less: don't hesitate to discard some of your investments if there is no more clear business value in them.
This one is a bit more difficult and less obvious than the rest. By introducing uniformity on the outside (standardization at the interface level) you make interoperability easier, because more diversity increases a learning curve which reduces the achievable reuse. This is because when things get harder (to comprehend), the potential for reuse decreases. Having said that, internally, anything should be possible. Diversity should be allowed to be able to deliver effective core service logic. Effective core service logic contributes to the attractiveness of services and would inherently increase reuse potential. The one size fits all approach does not work on the inside as well as the outside. Some balancing and fair trade-offs may be required to make it work.
Perhaps it's wise to make the following statement: Thomas Erl already described the principles for service orientation. The statements in these posts do not attempt to redefine these principles. If you want to review them, take a look at the soabooks.com website where you should find lots of information on the Prentice Hall SOA Books by Thomas Erl et al.
Phew... this is it for now. I hope this gives food for thought. If you agree or especially if you disagree with the statements made here, please be invited to drop me a line and explain your point of view. Perhaps I have to change my insights and way of thinking :)