Semantics

I don't think humanity has a good handle on semantics.
The case frame thing (from Filmore) is a nice workable proxy, but we don't really know how to represent meaning.
Bag of word techniques (e.g. latent semantic analysis) are a nice proxy and enable the extraction of some semantics quite automatically.
Even simple problems like the meaning of the relatively unambiguous sentence "I went to the store." are open to a wide range of discussion.
Problems like what is a word sense, metonomy and metaphor are all difficult for machines but easy for people.
That's not to mention default reasoning, gricean implicatures, tone, gesture, and common sense reasoning.