Great question! From the top of my head in no particular order:

- It is reliable

- It is focussed

- It models its domain well

- It is efficient in its interactions and use of resources

- It scales the patterns of interactions to the proficiency of its users

- It is fast

- It does not break compatibility of its interface without reason

- It is intuitive

- It is well documented if documentation is necessary

- It is accessible and international

- It is open (source)

- It is extendible

- It has a community around it

- It helps users to understand the domain and possibly extends the common understanding of that domain

- It works in the best interest of its users

- It does not lose data

- It fails gracefully and explains reasons for failure clearly

- It does not contain ads

- It fosters creative solutions to problems