Americans and other aliens should also note that -- while "it's pants" is an expression of disapproval -- "it's the pants" is short for "it's the ants' pants", an expression of approval.

We have the same thing here. Fairly recently, "It's The Shit" has become an expression of the highest approval. You wouldn't necessarily use it to describe another person, but you certainly would use it to describe a nice gadget. The car player, for example, is definitely The Shit.