What are you waiting for?
Definition 1 of 2
Pro Tip 1/3Prompt Action
Use it to encourage someone to act quickly, without delay. 
The taxi is here, what are you waiting for?
Pro Tip 2/3Casual Tone
It's conversational and friendly, but can be demanding in certain tones. 
Come on, the movie's starting - what are you waiting for?
Pro Tip 3/3Expectation
Implies the listener knows what to do next but hasn't acted. 
Your favorite song is on, what are you waiting for? Dance!