It also helps that the universe feels like a living, breathing universe thanks to the successful collaboration of writers. There are only 5 episodes of straight-up overarching plot in the entire series, leaving the other 21 episodes to expand on character development, explore interesting aspects of this universe, and just have fun. There's only one episode in the entire series that doesn't work at all (episode 21, if I recall the numbers correctly; Boogie Woogie Feng Shui), but every other episode more than makes up for it.
Also, while everyone mentions the soundtrack by itself usually, it's great to see how the music and direction are completely linked. If Cowboy Bebop didn't have the music and the animation direction to go along with the music, it wouldn't be the same. Since Kanno's score adds so much to each of the individual scenes.