Beyond Selidor

Agreed with all of these (although I haven't seen Being Human). Veronica Mars never could quite hit the heights of season 1 again. The Musketeers had such a noticeable tonal shift that I never even bothered watching season 3. I liked season 3 of Broadchurch but it wasn't something essential. I think Pretty Little Liars is the one that frustrates me the most. The first 2 seasons were so enjoyable. Even the stuff I wasn't fond of - namely the Aria/Ezra relationship - at least had some kind of stakes.

My most recent cancelled too soon show would be Paper Girls. Amazon never even bothered to promote it, I only found it existed by accident.