Do I smell the free-will argument insinuating its invidious way even here ?
The thing I most wish I had thought of, myself, in re submitting things to editors, is the person who sent their manuscript a birthday card when it had been in slushpile a year; I heard about it from the editor in question who was tickled into looking at the thing shortly thereafter.
..also, I wish I could write short fiction. Or rather, I wish I could write halfway decent short fiction in any less time than it takes to complete a first draft of a novel. I greatly respect people who can write a thousand-word story in the same sort of timescale it takes to write a thousand-word chapter, but that's not a skill I have.
Pure, utter rationalization I'm afraid.
The birthday card is brilliant. I'll put that on the list of things I wish I'd thought of, and hope to never need to do. I'm actually doing submissions based on turn-around time rather than rates because the single thing I need most is feedback.
Short fiction is a pain. I have trouble telling anything meaningful in less than four or 5,000 words. I get that it's intended to be conceptual, crisp, etc... but my love is in worldbuilding, and character-driven material. Something which, at least for me, takes a bit more space than the typical 3 or 4K limit allows.