What makes a good Product Owner?

Kanban Jedi put up a good blog post about Product Owners. After a little banter back and forth, I figured I had a few words to add myself:

• If you have a single customer, the mature agile team doesn't need a product owner. Their customer is the product owner. Direct communication is critical.
  
• If you have many customers (for example, 6 beta customers and a market potential of 100's), then you need a product owner to go out and be the liaison between all these opinionated parties. It is the product owner's responsibility to find the core set of requirements for all customers without including requirements unique to a few.
• Your product owner should understand technology. Prioritization of the backlog is greatly dependent on understanding the cost of features which includes the complexity of building them. A basic understanding of technology will aid in this discussion. If they have implemented a software system, then they at least have an idea of how complex it can be.
• Your product owner shouldn't represent the techonlogy, but instead represent the domain. They represent the customer. The customer doesn't care what language it is coded in, or how cool the algorithms are. They want something that meets their business needs. Make sure the product owner is responsible for the domain first and foremost. They should not be the architect or tech lead or anything close to that role.

Anything you would like to add to the list?