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.
Crowning Moment of Heartwarming: Mariana is pleading for her new husband Angelo's life, and asks Isabella to get on her knees before the Duke to help her. Angelo tried to rape Isabella, broke his promise to spare her brother in exchange for sex, and has been Isabella's enemy for the whole play. Increasingly hysterical, Mariana begs Isabella to get on her knees, and none of the bystanders think she will...And then Isabella gets on her knees beside Mariana and begs for Angelo's life despite all he tried to do. A beautiful example of the Christian mercy that as a future nun (maybe), Isabella should be practicing.
What makes this doubly heartwarming is that she now shows genuine mercy, not the cold, distant mercy she showed her brother earlier.
Ensemble Darkhorse: Mariana has had a great deal of Romantic art composed for her, including a famous poem by Tennyson.
Strangled by the Red String: Some see the Duke's proposal to Isabella as this, seeing as how their interactions up to that point have no romantic chemistry whatsoever, and it almost seems like Shakespeare addressing the awkwardness of a Pair the Spares happy ending.
True Art Is Angsty: This is one of the darkest of Shakespeare's comedies. But it's also one of his less popular plays. Perhaps that fact averts the trope, ultimately.