True Blood
First two seasons were fucking great and the third was pretty damn good but from there on the show turned to shit and never recovered. From bad pacing, side characters getting way more time than they deserved (even half of an episode was dedicated to a funeral for one of these characters) and even the main story lines being poorly written at times and having little payoff. Also in Season 5 they falsely advertised what a story line would be.
I also watched Season 8 of Dexter…This one requires no explanation but I should have stopped with either Season 4 or Season 7.
I watched both True Blood and Dexter until the end, after I had stopped enjoying both of them. Dexter started sucking as soon as Julie Benz showed up.