Don't worry about acceptance criteria!

What? What did you say? Don't worry? If I got no acceptance criteria, how should I be able to test it then? And what about the developers? How can they build something without knowing exactly what they should build?

Hold your horses.

I said, don't worry. Really, there is nothing to worry about. Why? Because you took your time to refine well. You know what to build. You spoke about it and asked questions about it. You even gave a suggestion to do it a little bit different and while doing so, the customer would be better served. You developers have investigated the code by getting a resource. Someone who knows how the current situation is. He gave you anchors where to begin and how the architecture was build up and why decisions where made.
The product owner has explained what the customer wants and why he or she wants it. You know the benefits of the solution. You all together created some scenarios, just to get a good feeling about it. And those scenarios helped you to find a direction to go to and which way you should not. You triggered someone to say something no one had thought about. You eliminated the questions and a whole lot of uncertainties.

Exploration

So why worry for somewhat incomplete acceptance criteria? If developer and tester, keep up together on the user story, the acceptance criteria will grow by itself. And in stead of having them written in stone, you'll find a much nicer way to define your story. You explore during development what the possibilities are. You make sure you get constant feedback from the product owner or the key user. You find out all together what you're really going to create!

Be happy

And when the story is implemented, you are very, very sure that you have all the acceptance criteria you want! So, don't worry about the acceptance criteria, and enjoy the exploration during your implementation. It's going to be fun, mark my words!