Castle is a great television series. It's not just a great police drama/murder mystery series, it's a great series, period.
But what makes it great isn't the plotlines for said murder mysteries. No, the plotlines are usually average to slightly above average for the genre. They're effective, but not earth shattering (although on occasion the show has risen to some rather high levels in the plot department). Nor is the presentation of the police detectives and how they do their jobs what makes the show great. No, what makes Castle one of the most entertaining television series of the last decade are the great characters and the interaction between them. Key to that interaction is how the actors bring those characters to life.
Nathan Fillion was a star waiting to happen. His performances as Malcolm Reynolds on Firefly were fantastic. He has charisma and charm and a sense of wit that is second to none. The fact that Fox botched the promotion of that series (and aired the episodes out of their intended order) just speaks volumes about how incompetent some of the people at that network were. (Beyond Fillion, the show was written and occasionally directed by future Avengers writer/director, Joss Whedon.)
On Castle, Fillion finally got a chance to find a wider audience, and it was the perfect vehicle for his acting talents. But even Fillion alone can't carry a show without other talented cast members to play off of. Fortunately, Castle is loaded with talent, both in front of and behind the camera.