Good SOA is the one that helps you solve your business problem. Good SOA is not about ESBs, BPM or registries, you CAN solve your business problem without them. Of course, if an ESB helps you implementing the solution that solves your problem more efficiently, by all means, you should use it. But is ESB a must have? Not at all. Same is true for BPM/BPEL products or even registries. These products may or may not help you build your solution faster or make it more flexible. It all depends on your business domain, your specific requirements and your existing IT environment.
Good SOA should help you build solutions faster. There should be a measurable improvement of developers' productivity. This means that it should be easier to build solutions with SOA than without. If this is not the case, if, for instance, developers complain that they need to jump through all sorts of hoops to invoke a service, then something is wrong with your SOA.
A corollary to that is that good SOA spurs innovation. Just look at successful public Web services, such as Google maps or Amazon. They triggered creation of applications that developers of these services had never envisioned. Note that true innovation is almost always grass-root. Your SOA governance processes must support, not prevent, grass-root initiatives. When you build a good service, you really can't envision all possible usages of this service. So let others help you.
What about flexibility and extendibility? We're often told that good architecture should be flexible and extendable, so that future requirements can be implemented without major changes to the architecture. This is one of the most cited SOA benefits. But we should not go overboard trying to implement ultimately flexible solutions. Nobody can predict future with certainty. No architecture can accommodate ALL future requirements. Besides, oftentimes flexibility increases complexity. More complexity − more difficult it is to deal with the architecture, so developers' productivity plummets. So focus on what you know and build the best solution for the task at hands. Chances are it will be flexible enough.
Once again, good SOA is not about technologies or products. It's not about WSDL, SOAP and WS-*. You can build great stuff with just "Plain Old XML" over HTTP (and you can build equally great applications with WS-Security, WS-RM and all the rest of WS-* soup − but make sure you have proper tools in place to deal with those). Just make sure that your architecture solves a real business problem and that it makes developers more productive.