a lot of these responses assume it's just TOS violations but they reject apps for many other reasons.
i am a professional artist and i have made a bunch of very simple websites as part of my art practice - a domain with a simple image or animation. i made an art app for the iphone along these lines.... apple rejected it outright because it was too "minimal" for their tastes.
this frustrates me to no ends... who are they to say what my art should be? that my work needs to be complex. clearly this has made me less keen on it as a platform, and many of my artfriends are second-guessing making apps.
another example - an app i developed for a client - needed rating because there's a video of someone saying "girl, you looking good". this is apple's notion of what mature content is. they're pretty over the top, they make tipper gore look like g.g. allin. what makes matters worse - it took them FOUR WEEKS to tell us this. and we have adjusted the rating and have now waited another TWO WEEKS and they haven't reviewed it again yet. this is crippling to a business. this is not internet time, this is boat-trip-across-the-atlantic time. very frustrating.