Dear Editorial Anonymous,Nothing.
Can you translate this from editor-speak to writer-speak? What does it mean when an editor tells you they're turning down your manuscript "with regret"?
Listen, I understand the impulse to parse rejection letters, as pointless as it is. Look, they aren't a pan of gravel in which you might find a flake of gold if you examine every piece of grit. They're hastily-written letters, so what's there is there.
Think of it this way:
option 1: The editor really wants to get something across to you.
Editors are verbal people. If they want to communicate something, they will.
option 2: The editor is not trying the get something across to you. This could be because she just doesn't want to go into why she's rejecting the manuscript, or because she's actively avoiding being blunt enough to tell you why. Either way, editors are verbal people. If they want not to communicate something, they won't.