In addition, an E3 does not allow "dual intent". i.e. you may not seek permanent status in the US while holding an E3 visa. This means no Green Card applications, including the Green Card Lottery.
One other thing that may not be unique to the E3, but is worth understanding is that your tenuous presence in the US is bound to the employment quite tightly. If you quit your job, or are let go (startup winds up, etc), you must exit the US within 10 days. That's 10 days to pack up your things and leave the country.