If you're expectant, you're waiting excitedly for something. A throng of expectant fans might wait outside a concert venue, hoping for a glimpse of the band arriving.
When someone's expectant, they are anticipating something good, eagerly awaiting it. When you're hoping for a call, your expectant glance might land on your cell phone every five minutes, and if you're especially hungry for lunch, you might keep an expectant eye on the clock during math class. Because pregnant women tend to be excitedly waiting for a baby to be born, they're also commonly described as expectant.