This one's a very odd choice for a season finale, as noted elsewhere. Ted Levine is absent, although this gives Jason Gray-Stanford a little more of the limelight it establishes both that he's a competent (if mediocre) police officer in his own right rather then just comic relief, and shows that he has some faith in Monk too.
Sharona is here mostly for comic relief and to get the ball rolling - her treatment of Monk here borders on malpractice. Other then the running gag of another failed relationship, she doesn't do a lot here.
Given a relatively uncharismatic bad guy, that gives us the "stunt" guest stars - Brooke Adams (Shalhoub's wife) as a put-upon flight attendant, Tim Daly for his debateable value as a former Shalhoub guest-star, and Garry Marshall playing a reasonably likeable character. Mr. Marshall has played more obnoxious characters in recent years (see his gig on Murphy Brown) so he actually comes across as kinda sympathetic toward Monk here and with Sharona off doing her thing he's Monk's only ally for most of the show.
So that means much of the story is on Monk and...well, it gets a bit tiresome after a while. If the producers, director, etc., wanted the audience to feel what it's like to be trapped in a plane with Monk for hours on end like the detective's fellow passengers, they succeeded. It's really, really annoying. A little of Monk's illness goes a long way, and while there are some funny moments (his being "trapped" by the little girl), for the most part the illness stuff here is portrayed as annoying mannerisms rather then lovable character insights.
The crime itself is pretty straightforward and it's kinda interesting to see how Monk figures it out...the first couple of times. After that, it begins to get repetitive. We know who the guilty party is - Monk knows who the guilty party is. None of the evidence Monk comes up with "proves" anything to anyone else. So why do we have to sit through yet another revelatioin?
And in the final act, Monk framing a pilot for drinking to get the murderer captured is...well, kinda criminal in its own right.
Overall, some interesting bits and gags, but overall this one just seems to sit there - it'd be one of those USA website one-minute mysteries without the "special guest stars" and the overdone OCD jokes.