These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Rizzoli & Isles
Adaptation Displacement: To the point where Gerritsen's latest books have "Rizzoli and Isles: Book title here" instead of just the title itself. Also, note that there is no separate section for tropes in the main page that belong solely to the novels, unlike the page for Dexter.
Complete Monster: Charles Hoyt. He's a unrepentant sociopathic serial killer who tortured and killed many people. In particular, he preys on Jane and threatens to rape Maura. Also, much of his behavior is uncomfortably reminiscent of rape.
Crosses the Line Twice: In the second episode, an old lady insults the aged detective who comes by to ask about her son, due to their history together. Then she calls Rizzoli a "greaseball dyke cop". Then she uses a sardonic tone when addressing the black Frost as one of those "Affirmative-Action cops. Which island did you come from? Do you speak English?" This all happens in the space of about two minutes. Oh, and she's clearly Irish.
Die for Our Ship: To the point where any man put in the way is ruthlessly run over. Casey is a prime target of this.
Genius Bonus: Whenever the Boston skyline is depicted, the weather beacon on top of the Old Hancock Building will match whatever weather might be shown in the following scenes. (It'll blink or shine steadily in either red or blue; the color patterns and their meanings are well known to Boston residents.)
Harsher in Hindsight: Season 1 episode "Born to Run" has Jane and Maura competing in the Boston Marathon and getting caught up in a case where two runners are shot dead in the middle of the race. Two years after the episode aired, the Boston Marathon was the target of a terrorist attack that killed 3 people.
Hey, It's That Guy!: Anyone watching the show who grew up on 90's Disney Channel will probably do a double-take upon realizing Jett Jackson is one of the cops.
Anyone who watched MacGyver will likely recognize Bruce McGill as Mac's ne'er-do-well friend, Jack Dalton.
Just Here for Godzilla: Some fans of Rizzoli & Isles may be attracted to the show because they are fans of Angie Harmon and Sasha Alexander from their old shows. Based on the ratings, it may now be a case of Come for the X, Stay for the Y (in this case, come for Harmon and Alexander, stay for the quality of the show).
Moral Event Horizon: A variation. Paddy Doyle was a ruthless mobster who had standards (no women in children on his hitlist). However, as "Dance with the Devil" reveals, he was personally responsible for the death of Cavanaugh's wife and kid, effectively breaking his personal code of conduct.