I'm curious to see what is safe and not safe to disable. I'd like to disable gmail but when I try to disable it, my S4 asks me if I want to replace it with the stock version. Can't get past that point. I use CloudMagic for email.
Disabling Gmail also removes the updates it has received through Play Store (so it's back to it's factory reset level), thats what that warning means
I have the following disabled. Email (I use Gmail), Home Screen Tips widget (gingerbread holdover for some reason), and Messaging (I use Hangouts)