What I think is really important (and would make Redox succeed over the other OSs - they are basically what Linux doesn't have and what made it more something for advanced users than for general usage):
Another part of being easy: It needs to look good and simple. This is more important than many people realize. You won't use an OS that looks like the design is a couple years old, and you won't use it, when the UI is not intuitive and compact. Redox doesn't need to develop a new design policy for this, using something like the Material Design by Google would even provide a better UX because the design is consistent and known to the user even on different devices and OSes.
Make everything nice and UI-Based and easy to use. If the user wants advanced stuff, they can get that, but things like the need to use the console as a normal user on a regular basis scares many people away.
Another point for security: Use build-in encryption and security methods whereever possible. Have them always activated if they don't need any userconfiguration (they run unnoticed for normal users in the background) and the techniques that require the user to behave in a different way they would without need to be moved to the "advanced" part.
Look at what works and what doesn't on other OSes. Something I really enjoy using Android is the permission-management. Every application needs to ask for its permissions and they can even be granted or denied at runtime - no popular desktop OS has that, the only options are "run" and "run as root/administrator" and then the application may do whatever it wants to.
(This one is in brackets, because it is not that urgent - but would be really nice.
Make Redox easily extendible. Custumization will be important to many users - and when there are already the mods X and Y present because Redox can easily be customized by modifications and they maybe are even drag + drop - that would be amazing).
This one is probably the biggest: Compatibility with programs from other OSes. One of the biggest downsides of Linux is that you can't use most of the Windows programs user X is used to on it. It takes a lot of time to get into the new system, and many users don't want to have that much efford when they switch. We can eliminate this problem by making it possible to run (mostly WIndows, but also Mac and Linux) foreign programs on Redox. The easiest way to do this would probably be a mini-vm for each program. This is a lot of work, but I can tell you... If Redox gets this feature, it will be over the news (and get a bigger part of the market) in no time!