Here are possible reasons (unofficial, from here):
Vibration. It is not permitted to use continuous vibration in your apps - short bursts as warnings is all that is allowed. Don’t bother trying to set up a timer to keep the vibration going, it will cause your app to be rejected.
Linking to private frameworks. This is obvious, but somehow in playing around with stuff we had linked to the MoviePlayer.framework. That’s a no-no, and cost us about ten days while we unlinked that framework, recompiled, and then resubmitted.
Improper handling of editing in tableview cells. Also obvious, but be aware that if you enable table cell editing, you’ll have to manually specify which cells should respond to editing controls and which should not. We had some random prefs cells in one of our early apps that were able to be swiped to bring up a ‘delete’ badge. Of course it didn’t do anything, but Apple justly considered this poor design and rejected our app.
Icons. Make sure the 57 pixel icon is identical to the 512 pixel version. Also, use a different icon if you are creating ‘lite’ and ‘pro’ versions of your app (i.e., free and paid). Using the same icon for both sends your app straight to … you guessed it … the bin.
Copying existing functionality. This one is much more subtle and insidious, and has probably affected the great percentage of developers. In addition to the widely publicized Podcaster debacle, reports from user comments indicate that Apple is casting a wide net when looking for duplicated functionality. Mini web browsers, or apps that essentially show web pages, seem particularly vulnerable, even if they add new and/or useful functionality. Stay away from email clients as well.
Using appropriate keyboard type. If your app asks for a phone number or other numeral-only input and you present a keyboard that also includes the possibility of entering standard alpha-numeric input … yep. (Thanks Jeremy1026)
Version numbers. If your app is currently at version 0.99 or below, you’d better consider giving it a promotion as Apple seems to prefer 1.0 and above. One of ours was recently rejected for being .016, with a message suggesting that our version number wasn’t even numeric. When we resubmitted the same app from scratch as version 1.0, it went through.
Network Reachability. If your app requires any type of network access you need to make sure it works when that access isn't available. If it doesn't it will be rejected. Apple provides sample code to test this which you can use as-is in most cases: https://developer.apple.com/library/content/samplecode/Reachability/Introduction/Intro.html
And last, but not least:
- Flatulence Don’t even try. ;-) UPDATE: sorry, this seems to be outdated by now. Apple makes a lot of money now with "fart apps": see this article.
Edit:
Here is a link to a recent article about ten iPhone Apps That Didn't Make Apple's App Store.
And a tip: Apple has a Mac app called Application Loader that you could install. Once you install it, it analyzes your app's zip file. It verifies all the certificates, icons, and other things are correct before submitting to Apple. Using the Application Loader minimizes your chances of app rejection.
Another interesting resource: App Store Roundtable: Transparency and the Approval System (appleblog.com)
Yet another edit:
New rules by February 2010: "No Swimsuits, No Skin, And No Innuendo" (source: TechCrunch article, Wobble author's blog)
By the way: during the iPhone 3.0 preview event (march 2009), an Apple spokesman told that 96% of all submitted application were approved.
