Earlier quoted context omitted.
Photoshop will save in the original format unless you do something to the image that isn't supported in that format (e.g. adding vectors, layers, layer filters, etc.) Then it will switch to .psd, although you still get the option of saving in the original format with Save As. I think that is a good design.
What would be better is having a dialog that says "You've used some additional features that can only be saved in .psd format: Save as PNG || Save as PSD || Save as something else" with the first being the default for when you press ENTER.
"%s may contain features that are not compatible with %s. Do you want to save the document in this format?
To save, click Yes. To preserve formatting, click No. Then save a copy in the %s format."