I think Mark Harmon did a wonderful job portraying the emotions Gibbs is going thru in this episode, but I think the storyline wasn't very good. The characters we have got to know so well over the last 7 seasons would not have let Joann get away with the murder. It was planned over a very long time... The connection between the killing of Shannon and Kelly & the naval officer (Joe Norton) was weak, if it was even there at all. I can totally understand the motivation of what she did, but not sure if I believe the characters would allow her to get away with it.
Yeah, not really impressed with the storyline of this episode. Either it needs fleshing out a whole lot more, to make a little bit of sense... But no matter what - I think Mark Harmon did a great job. As did Sean Harmon too! :-)