The biggest challenge is providing all the functionality that users want with a clean UI.

There's a trap and tradeoff in making an interface too clean: discoverabilty suffers.

Some users might never go beyond the simplest of use cases, but more advanced things need to go beyond that and it shouldn't be hard to find out how.

If users have to google how to do something, you have lost.

Apple is a good example of this. They value their clean and intuitive interfaces.

But I don't think they actually are. Give an iPhone to someone who hasn't used it in a while and see how well they handle things...