James Allwort of the Harvard Business Review eloquently explains why Apple's Siri is revolutionary:
[...] until now, nobody had put speech and artificial intelligence together in a compelling way: which meant that the voice systems on our computers and our phones have been clunky to the point where it was just easier to avoid them.
That's what Apple has fixed. Rather than simply roll out technology for its own sake, Siri starts with a deep understanding of the job users have for their devices -- and then deploys speech and AI technologies in a way that actually helps them accomplish what they're trying to do.
Turns out that Siri isn't without a sense of humour either, as the Shit That Siri Says Tumblr blog demonstrates, which is where the above image was lifted from.