A "letdown" is something that wasn't as good as you expected it to be. Here are some things that might be letdowns:

• a movie that looked good in previews, but was boring when you watched it
• an album by a singer that you used to like, but who has now changed her style
• a restaurant that you read good reviews of, but didn't like when you went there

The phrase "___ was a letdown" is a little casual. A more formal alternative is "___ was disappointing."