Shows that started off great and then fizzled out.
I find it sad when a series is new and exciting but as seasons go on, they get let down by bad writers and stale ideas, falling into a rut of being the same thing over and over.
I'll start off with Person Of Interest.
Great when it started but got so boring in the end that every episode was just look! bad guys! run away and hide somewhere, have a gunfight with the bad guys wherever they hid and then make it back to the hideout.