There's a lot of things that are obvious when stated, but for various reasons people don't really grasp their ramifications. Put another way, it seems often difficult to grasp just how significant an already superficially internalized concept really is.

E.g. pigeonhole principle -> no perfect compression (among other similar proofs)

Another a favorite one I have is understanding just how "weird" it is that pi, a constant, exists. Take a rope and tie it around the Earth so that it fits snugly on the Earth's surface. How much more length of rope does it take so that the entire rope circling the whole of the Earth can be lifted 10 feet off its surface? ~31 feet (this was at least surprising to me the first time I was asked this question, which was long after I first learned about pi).