Ask HN: When to launch your app?

So I've often read that you should launch a product as soon as possible. Get it into the hands of your customers and iterate/iterate/iterate. How do you determine what is the MVP of your product though? My app for example, might already be able to do XYZ, but I'm still working on ABC which set it apart from the competition. Personally I feel like ABC need to be done before I can launch, but it seems as though everything I read says to get it out once XYZ is functional. If you've launched an app in a highly competitive space, how did you decided when to release your product?