UDIDs were meant to be anonymous, but developers use them to keep track of the usage of their apps and to help target advertisements. The problem is that with a little more data, UDIDs can be traced back to a specific user. In fact, back in 2010 a study showed that 68% of the top apps in the App Store transmit the UDID of each device that downloads it. One app, CBS News, also transmitted the user's name along with the UDID.
Advertiser Identifier which users can change or opt out of. When the May 1st deadline arrives, the new Advertiser Identifier will have been out 8 months, enough time for developers to figure out how to use the information to track their app.
Besides rejecting apps for UDIDs, Apple is said to be looking to turn down apps not optimized for a Retina display. Additionally, apps for the iPhone must be written for the 4 inch screen of the Apple iPhone 5.
source: 9to5Mac via GIGaom