A user interface is like a joke
Heads up! This post was written in 2014, so it may contain information that is no longer accurate. I keep posts like this around for historical purposes and to prevent link rot, so please keep this in mind as you're reading.
When I hear a simile like this, it's hard to not share. It's not so much an analogy as it is a good rule of thumb:
A user interface is like a joke. If you have to explain it, it's not that good.
And I've heard some websites tell pretty bad jokes...