Since the inception and launch of the Apple App Store Apple has amassed billions of downloads across the hundreds of thousands of apps. While most developers haven't had issues getting their apps approved some notably have and typically those rejections come with little to know explanation as to why.
Thankfully, Apple is a bit sympathetic to the plight of its developers and as such has published a list of the most common reasons their hard work seemingly went in vain and was dumped to the cutting room floor!
Luckily for most developers the main reasons for rejections are simple and can easily be remedied. The top reason stems from administrative issues. If devs fail to provide enough information or a valid demonstration account, then their work will simply be ignored. Buggy code and none compliance with the developers agreement also top the list.
The rest are seemingly ordinary rejections such as Apple wanting only polished submissions that aren't in beta or demo testing, or app that contain false, fraudulent or misleading representations. Ironically the one rejection we don't see listed here, and one Apple has come under fire for in the past, is the duplicate content requirement. Apple has often come under fire for removing apps it feels duplicate any service or app that the company itself already provides.
For those looking for more resources and a list of guidelines used to review apps submitted to the App Store and Mac App Store, Apple offer this App Review page.