Clearly we must live in a different social setting, never seen a custom cover (the actual casing of the phone to be precise) on an Android phone. And there are many of those in my circle of friends. Maybe they're popular with teenagers. Protective covers was all I found at dealextreme, and you can get those for iPhones as well.
Custom ringtones and logos/wallpapers have been around for over a decade. There's nothing new about them. It's a huge stretch to call them "killer features" in 2011. As for stock ringtones, I still hear the Nokia tune many times per day, and the owners of those phones are aged anything between 20 and 100. Some people just don't care.