First two seasons were phenomenal. From that point forward it was a battle royal between the writing team, with each one trying to push their personal perspective on the series to the point it was like watching three different series mashed together.
Spartacus
The lead actor died at the end of season one. That is it. Pack it up and go home.
Prison Break
You had one good season. It was the concept. The moment the actual break is done, there is nothing to go on.
That's enough for now.
Grey's Anatomy
Because just how long can that thing go on for?
Law and Order
Any of its iterations. Same as above.
House M.D.
Good show until you start to put in and take out characters just to maintain the pulse of the thing.