When you had high expectations for something, but it disappointed you, you can say "That's a letdown."

A "letdown" is something that's disappointing. for example: 

Is it just me, or was that a bit of a letdown?

The phrase "That's a letdown" is pretty specific, though. You say it when something has just disappointed you, and you're feeling a little sad.

For example, you might say this if you read in the newspaper that your favorite bookstore is shutting down, or if your daughter tells you that she didn't get into the college that she applied for.