It's popular because the actually developed the back stories for the characters in a non- contrived way. not forced, but as part of the episodes. in short ,it was well planned out and so we are invested in the characters. the stories are a bonus- they also integrated the new characters(ziva,vance, et al) better than other shows.
NCIS is my favorite TV show, just as JAG was before that. I think that the most important part of this series is the character development and interaction. It's a solid show, the characters change over time, it's not the villain/case/thrill of the week, but entire character arcs that span over seasons. Then again, as Bellisario (the original creator of the show) has.....