Hmm. If I say "Everything can have a poem written about it," that seems to actually encompass everything, doesn't it?

Nope. Writing a poem about something pretty much means 'describing' so then you run into Schroedinger's cat and the uncertainty principle and that sort of thing. Or to give a simple exception, you couldn't write a poem about an idea that has not been expressed yet, or a phenomenon that has not yet been observed.

There are probably plenty of other exceptions. Everything is just too broad a concept :o)

Besides, why aim for everything? Half the fun is in defining.