I would say it is mainly the worker management and sophisticated device monitoring. You can set your own alert limit for each worker, receive alerts via e-mail and/or mobile app and view a history of monitoring events on the website, so you can go back and look what and when went wrong.
You also have an option to lock your payout address (unlockable by wallet signature), so your coins are safe even when your account is compromised. We have two-factor authentication (OTP/U2F) too. There is also an activity log listing all the important changes made to your account.
But in general, I think it is the clear data presentation what users like the most.
It is not that easy to describe, trying it yourself is always the best option.