Most people's complaints about Siri as far as I can tell (via Twitter mostly, so it's a tech crowd) is that it's basically not Google Now. That is, Apple doesn't have a search engine so it can't answer general interest/trivia questions unless it's specifically related to a domain Apple has included (such as Wolfram Alpha or MLB for baseball).
Now many people think it should be able to do that, but that's certainly not what it was designed for. Siri is a "personal assistant" to help you do things on your device via voice. They've recently begun to expand these integrations by creating a Siri API which will probably expand to cover more domains over time.