What a ___" communicates the idea "That's really ___" or "You're really ___." Here are some examples:

If you have to throw away food that's still good, you can say:

What a waste, huh?

Here's something a woman can say when a man opens a door for her:

What a gentleman! Thank you very much.

If a lot of people show up to your event or party:

What a great turnout!