They explicitly want people to try again, some of their biggest successes were accepted "eventually".

    Many of our successful founders applied multiple times 
    before we funded them (including Drew Houston of Dropbox).

http://blog.samaltman.com/applying-to-yc

I think they would be very hesitant to call any ideas terrible - recently they instigated changes here in comments outlawing "gratuitous negativity" and one of the stated reasons was they don't want to risk people abandoning ideas because of criticism -

    New work and new ideas are fragile. Too much gratuitous 
    negativity might be the difference between someone giving 
    up on a crazy idea and building the next Airbnb.

http://blog.ycombinator.com/new-hacker-news-guideline

Rejections won't always be actionable reasons or even you/your idea/your team, you're also being measured against everyone else's applications and YC's collective experience and knowledge of ~700 other startups.