They've had 2 years since iPhone 4, so the expectations are pretty high, and most are expecting something jawdropping. This is the double-edged sword with being an innovator at the level that Apple has been over the last 10 years - people's expectations get bigger and bigger, possibly to unrealistic levels.
To be honest, I can't imagine what else Apple can do that would blow my mind... fingerprint scanner? Breathalyzer? If it's just another "faster processor, better screen, more flash memory", then people like me are going to be sorely disappointed. I'm hoping for the best though.