I responded for some clarification: Where could I improve? What would make me a more attractive candidate? Nothing. We're a pretty close-knit tech community (Boulder) so it felt like a real slap in the face.
I manage dev hiring at my company, and when I give take-home assignments, I make sure to give them a timeline of no more than 2 hours. If you get to that point, it's okay to tell me what you would have done, as I should be able to tell by that point if you know what you're doing and could accomplish it.
"Busy" is not an excuse. If you've got the time to interview someone, you make the time to treat them respectfully and provide quality feedback.