For me it was "Eyes Without a Face" (1960). The synopsis was very intriguing and all of the promotional artwork was creepy, visually pleasing, and really cool. I love horror movies and I do enjoy horrors that can be a bit slow moving and psychologically terrifying. I was so interested in this movie and when I found it streaming I was excited.
I struggled through watching about a quarter of it before giving up. I wanted to love it so badly but I ended up being really bored with it. It was slow moving but this pace did not add any rising dread or tension, it was just slow. I'll try to revisit it someday but for know it was a bit of a let down.
What's a movie you were very excited for that left you deflated?