Some of the features can be downloaded in the Samsung App Store. There's a section just for the Galaxy S6.
Options I used in the past that are not present in the Samsung Galaxy s6 include:
1. Show only contacts with phone numbers in the contacts menu.
2. Control LED notifications beyond on and off
3. Holding down on a text message to add the number to contacts.
4. Cyclical scrolling through the app menu.
5. Having a button to bring up the settings menu in an app.
I found an app that sort of fixes the issue of (2) (Although not perfectly.), but the others are just annoying changes for which I have found no fix.
I'm sure that there are many more software regressions as well. I've read about a few of them that other people are having trouble with today. All and all the ability to customize the phone is decreased over previous versions.