Gilmore Girls and The West Wing are two of my favorite shows ever and they both became completely different after their respective creators left and they became sad shells of their former selves.
Brothers and Sisters lost quality when ABC kept cutting the budget.
The last half of the final season of Alias was absolutely terrible as they rushed to try to wrap up the story lines after it was canceled.
Veronica Mars is very sadly missed but I loved the movie plus iZombie is pretty close to being the same show but with a different cast and some zombies thrown in so at least there's some comfort in that.
Eyes with Tim Daly, Miss Match with Alicia Silverstone, and GCB (Good Christian Bitches) with Annie Potts and Kristin Chenoweth are just a few examples of new shows I loved that got canceled pretty much right away.
Trial By Jury was by far the best of the Law & Order series and they killed it off after only one shortened season.
The O.C. and Happy Endings are two of my favorite shows ever that got canceled while they were still good... now Looking can be added to that list and I hope HBO stands by their word and produces a final episode to wrap up the series.
On the non-scripted side, I really miss Tabatha's Salon Takeover/Tabatha Takes Over. I also really liked RuPaul's Drag U, DanceLife, and America's Best Dance Crew.
The Hills became terrible after LC left just as Laguna Beach became terrible after Kristin left.
Other shows I miss a lot (even though they "finished" their runs) are Sex and the City, Entourage, The Wire, Lost, 30 Rock, The Newsroom, and Friends. Arrested Development was also on this list but thankfully it was resurrected.
Besides Looking, the other shows I currently watch that I worry about are Nashville, The Mindy Project, and Marry Me because none of them is a big hit. The only show I watch that is popular is Scandal.
It is hard to give shows a chance when you're aware that they likely will just get canceled. Thankfully I don't own a TV anymore so I only watch a few shows.