Language is certainly our most versatile mode of communication, and
very effective at the same time. It is also heavily interlocked with
our elaborate social behavior that probably takes a sizable part of
those excess 500 g. I think that due to this, it tends to overshadow
in people's minds the huge import of vision to human cognition.
Looking at evolutionary history, visual processing is much older; and
it too benefits from our present abstractive faculties just like our
ancestors' vocal and sign communication have. So maybe the
conceptualizing powers that make us special have less to do with
language per se but imbue all of mind's workings?
That idea rather fits your view of mechanical speech processing as
"overwhelming" as it implies that to approach human ability there, a
machine would also need to get close to a number of other
superficially unrelated aspects of the human mind. In fact, this
stance is not uncommon among cognition researchers.
I think you will like this piece:
In the Wacky Protestor's return, he hijacks the popular
children's TV show, "Mr. Funky's Wild Time", and uses it
to control the hearts and minds of every child watching.