Newly introduced Reachability feature in iPhone 6 and 6 Plus bring all the onscreen elements closer to your thumb. Thereby, it makes it convenient for you to use bigger iPhones.
Reachablity feature may be very useful for the people with short hands or small fingers; it may be annoying for those who just don't want it for any reason. Is Reachability enabled on your iPhone and just don't know how to turn it off? Here is a quick guide:
Step #4. Scroll down to Reachability (under the Interaction header) and Turn it off.
That's all! Reachability has been disabled hence, now onwards it won't be activated just by double tapping Home button. In case, you want to enable it again, turn on the Reachability by following above steps.
So far there is no other customization of double touching Home button. Had there been any other as well, it would be nice. Of course, it may change in future and Apple may bring some more customizations. However, as of now, you would have to live with this only.
Just like this, there are so many other features which have been introduced with the latest iOS 8. Despite being great utility, they may not be convenient all the time. Take for example, Share My Location. Although it is quite good as it keeps your loved ones or the people who matter to you informed regarding your whereabouts, it may turn out to be privacy issue sometimes. So the thing is, no matter how useful something is, it may not be liked by everyone else.