Sarah, Head of Content at GDS, sent me this the other day. You should read it, it's good.
"There's not much here. Just words. And you're reading them. We've become obsessed with fancy designs, responsive layouts, and scripts that do magical things."
Go read it.
What do I mean by simple?
Think about the time you used the web and you just wanted to know the football score. You didn't want any flashy animations or videos, or any innovative UI, you just wanted information - fast.
Or the time you needed to know the opening times of a restaurant, or the postcode of that office you were on the way to. Think about when you used a cashpoint and you just wanted £20. You didn't want to check your balance, or print a receipt or win tickets to the World Cup. You just wanted £20.
That's what I mean by simple.
(I know that's not easy to achieve, which is why the principle is Do the hard work to make it simple.)