I think it would be manageable if they had a few generic templates. When they reject the app, have some check boxes for "Don't see the profit model", "Single founder", "Already exists", "Seems like too much of a 'startup idea'", etc. Those could generate a paragraph in the email.

Yeah, I agree. I understand it's probably not feasible when you have so many applications you have to sift through but highlighting key weak points would be nice.