I love the design of the iPhone 4, so I generally don't use a case (I do have one for specific situations), but I also understand the people who do use a case right out of the box.
Look, the iPhone is basically a glass sandwich. It has one more piece of glass than any other phone, but the part that you care about is something most smartphones share now, a giant touchscreen. You have to be careful either way, some people feel safer with a case, and that's fine, but others feel confident in their ability to care for the phone by itself. Boils down to preference.
As for screen protectors, I could never stand them. I like the feel of the glass (front and back) on the iPhone, and a screen protector just makes it feel... strange to me.