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The main characters are so endearing and so well-written that we care what happens to them in each episode. That's an important point: if the writing is good and remains relevant over time, a show can last forever. Especially if that show decides to branch out with different series of the franchise, as Special Victims Unit and Criminal Intent did.
I like how episodes are inspired by true crime stories that are currently making headlines (the episode "Selfish" takes cues from the Casey Anthony case). I like how a lot of the humor is comprised of "clean jokes," even when the situation is grisly. I appreciate how the perps drink Pathmark brand sodas while getting interrogated. I enjoy the crossover episodes. And I really like how the show is self-referential to previous episodes in the series, and also to other L&O series. Cross-over episodes rule!
It was upsetting to me when I learned that the original Law & Order series is to be canceled and replaced by a version taking place in Los Angeles. Gross. As if there aren't enough shows already with LA as the backdrop. Every episode of L&O was filmed in New York City, and New York City was one of the main players of the show-- that was Dick Wolf's intention from the start. To just up and fire the longest-lasting cast member of the show after twenty years is sad to me. New York is one of those places that everyone loves. It's teeming with excitement and hustle and bustle, everything is always open, it's full of history, and it has character. I think you'd be hard pressed to say the same of Los Angeles.