I definitely don't like to use a case on my iPhone 6 and my coming iPhone 6s plus. And one of the biggest reasons why is because I am a heavy user of swipe gestures in iOS and all the apps that supports it.
And the iPhone 6 and the new iPhone 6s has an optimized design to make it easier to do the left and right swipes that starts from outside of the device and into the display. The edges on the iPhone are rounded to make those swipes as easy as possible.
And by using a case on those very rounded edges, protrudes the display so that you get the case edge above the display, and that makes it harder to do those left/right swipe gestures.
And it's not only third-party cases that protrudes, also Apple's own leather cases for the iPhone 6 and iPhone 6s do that sadly.
So that is one of the reasons why I don't like to use a case on my iPhone. Of course there is a lot more reasons why, but this reason is a very important one for me personally.
Watch this video where I try to explain why:
Watch related videos from the Apple playlist.