It seems Google was tired of seeing all the ugly Android apps on their brand new Ice Cream Sandwich OS. And they ought to; Ice Cream Sandwich is a beautiful product on its own but when you launch a third-part app that does not follow the same design principles as the OS then it sticks out like a sore thumb and spoils the experience for the entire OS and not just the app.
So to set things straight, Google has now released an exhaustive list of UI guidelines for developers to follow so that their apps have the same consistent look and feel as the OS. This is the first time Google is doing something like this for Android. In comparison, both Apple and Microsoft have had UI guidelines in place for a long time now, which is why you see iOS or Windows Phone apps that look similar to the OS to the point where you can’t tell a third party app from those built-in. With these new UI guidelines, Google aims to achieve a similar consistency.
Having said that, Google has only provided these as suggestions instead of making them mandatory, so you will still find lazy developers not choosing to change their UI to fit the new look of Android. Hopefully though, the large majority will see the light and make the necessary changes.
Another thing to note is that Ice Cream Sandwich itself is available only on less than 1% of the devices out there, so developers currently have little motivation to modify their apps for ICS. It also remains to be seen how these modified apps will look on older versions of Android. Google needs to work hard to increase ICS usage on Android before they can expect the developers to create apps for it.
Google releases Android Design Guidelines for app developers