Mobile platforms. Just saying the phrase makes some people cringe due to the number of ecosystems out there. But with each ecosystem/platform comes a new UI. WPCentral has an interesting article how how close each platform’s icon size/shape really is.
It’s something you don’t really see until you line them all up. Also notice the absence of BlackBerry OS and Android. Both platforms have no standard/bounding shape for developers to use. While some people might hate the restrictions on icon shape and size I think it’s good to have when you want to really give the consumer the best UI possible.