Making something look simple but not boring, mundane or like a copy of something you saw 10 times today, is hard because it is highly depending on context and context is depending on understanding.

Facebook can get away with an f (and more so, it works for them) because it makes identification easy when it is ubiquitous – but that is not automaticaly the case when you have no brand recognition. The simple f might just be forgettable or, worse, remind people of something else.

There is a ton of additional caveats. That is what is hard about visual design: It is infuriatingly different even when you think it shouldn't be, every single time, and understanding both how and why it doesn't work – and I guarantee you, it will not work before it eventually does – is a recurring challenge for every designer, regardless of how simple the design or how experienced the designer.