Plot: Grey's Anatomygets dark this season. George is officially dead. Izzie leaves Alex. Richard grapples with his alcoholism. The finale involves a shooting. On top of it all, there are structural changes as the hospital merges with Mercy West, introducing extra relationship drama to the series.
Ending: "Death and All of His Friends" has to be one of the most heartbreaking finales, and it's as tense as a horror movie with everyone vulnerable to the hospital's biggest threat yet. Really, it's arguably one of the show's finest episodes. Reed and Charles come out dead. Meredith miscarries after Jackson and Cristina pull the plug on Derek in order to convince the shooter to leave. Richard talks to Mr. Clark, who ends up shooting himself. Alex, Owen, and Derek are hurt but come out alive.
Weepiness: Grey's Anatomy does its best melodrama in season six, so prepare for lots and lots of tears. You'll be getting residual tears after the George and Izzie's departures, but nothing can really prepare you for the devastating finale.