Monogatari series in general (besides Second Season which I really enjoyed), Hyouka, and Anohana.

For the Monogatari series and Hyouka, I bought into too much hype and was thoroughly disappointed. I can say the Monogatari series definitely has the potential to appeal to many viewers, and is great on a technical aspect, but just speaking from a subjective viewpoint, it's not my thing. Hyouka was just terribly boring to me and whoever the main girl was annoyed me to a great extent. It had its charm in the beginning but had too many boring arcs for me and I hated the ending.

As for Anohana, the series was too short for me to connect to any of the characters. Also, Menma annoys me. Characters I couldn't relate to + main girl I found annoying = not sad at all. As Anohana is one of those series that's known for being really sad, it just completely failed in that aspect for me, and thus, I thought it was fairly average. The ED is amazing though. I've listened to it so many times, along with transcribing my own piano cover for it.

EDIT: Remembered how much of a disappointment Chuu2Koi Season 2 was after reading @killjoy's post