Some people think that your usual language affects how you can thnik (http://en.wikipedia.org/wiki/Sapir%E2%80%93Whorf_hypothesis; not sure if I'd agree that strongly) - so possibly you wouldn't be able to miss expressiveness you're not familiar with (simlar to pg's idea of Blub programming language).
Wish I knew more about linguistics - but I think missing tenses or grammatic logical structures are more interesting than missing vocab - like for example some languages have a tense for describing things you only know about second-hand
http://en.wikipedia.org/wiki/Inferential_mood
(missed the bit I was meant to be replying to sorry...)
I think it's more about the relative ease or difficulty of expressing a concept in that language. If a concept is missing or unnatural, it's going to be harder to deal with that concept. Common examples of languages with more expressiveness than English are the multiple Russian words for blue, or the many Eskimo words for ice.
A thought experiment that I think works better is to imagine a language where people describe facial features extensively, to where a description in that language can easily be used to reconstruct an image of a person's face. Imagine how crippled a native speaker of that language would feel trying to speak in English, or the derision they would have towards the police sketch-artist process that took trained artists/interpreters quite a bit of time to extract a description from a witness that would sound like a 4 year old trying to describe a face. That's what linguistic differences are more like.
Wish I knew more about linguistics - but I think missing tenses or grammatic logical structures are more interesting than missing vocab - like for example some languages have a tense for describing things you only know about second-hand http://en.wikipedia.org/wiki/Inferential_mood
(missed the bit I was meant to be replying to sorry...)