Uffff, I have a lot to add in this thread, so let's start.
Gossip Girl - stopped watching around Katie Cassidy guest-starring and then I never really returned. Good decision.
90210 (2.0) - I think I was already done with the show by the end of season 2, however I do remember checking out the s3 premiere and then I decided this show wasn't working for me anymore. Especially when they've announced Jennie Garth was not going to be coming back anymore. Blergh.
Pretty Little Liars - this was always a little guilty pleasure of mine because it was SO ridiculous, however around s4 (when they've aired the Christmas episode) I decided I'll stop watching it now and just wait for the conclusion. And boy, was I disappointed with the whole "Who is A" storyline so I never returned to the show.
2 Broke Girls - this one really hurt because I LOVED both Kat Dennings and Beth Behrs, but the show was barely watchable and so I stopped watching at the end of s4.
Arrow - it's technically still on air and I do watch The Flash/Supergirl, so maybe I'll tune in back one day, but it quickly became the show where they've completely killed the element of surprise due to other characters dying and then coming back alive over and over again. The Vampire Diaries suffered from the same syndrom, however I did enjoy watching it a lot more than Arrow. Plus, Katie Cassidy, ugh.
Cedar Cove - first of all, I love me some Hallmark shows and movies. This one was incredibly dull but the scenery was fascinating. Would move to Cedar Cove in a heartbeat. The pacing of the show was really slow and I was done after s2.
Chicago Med - I still watch Fire/P.D. (although the latter is becoming seriously dull without Sophia Bush), but I couldn't get into Med at all. I've watched s1 and then sometimes around episode 5-6 of s2 just abandoned it. Still hurts because I love Colin O'Donnell.
Girl meets world - Boy meets world was a HUGE deal when I was growing up, so I thought this would probably work for me considering I was really interested to see how Topanga & Cory would work as parents to two kids. However, the cringe-worthy Disney-iness of the show killed it for me. I wasn't Disney's target audience, but I would have seen this one rather on a channel like Freeform or even Netflix. Too bad.
Madam Secretary - from an European standpoint, the show was very "America is the greatest country in the world and the rest of the world looks like THIS (insert stereotype)". I loved the cast, but the writing was just not working for me, I was rolling my eyes during most of the first s1 & s2.
The Mindy Project - I LOVED Mindy. LOVED. But the moment they turned Danny Castellano as a completely different character = jerk, I did not care for the show anymore. Plus, they gave waaaaaay to much of time to Ike Barinholtz's character.
Quantico - the show was terrible and I watched the whole s1 because I'm a masochist, I guess.