Apple has an amazing eco system, but it is closed and apps have to be reviewed, hence simple stuff can be flagged during the review process.

• When using Game Center make sure it’s enabled in your app not just the Game Center tab, also achievements and leaderboards need to be submitted for review.
• In-app purchases need to be submitted for review.
• When using external frameworks such as Chartboost, make sure you disable test mode when submitting for review as Apple considers it an unfinished app if they see anything saying testing like that.

More to come as we discover them, also feel free to post your own tips and tricks.