Feel No Pain: Diana has many complaints that her medication makes her suffer new problems each time it is altered, such as weight gain, sweating for no reason, constipation and nausea. However, after a while she claims "I don't feel like myself...I mean, I don't feel anything."
Averted when she throws away her medication in "I Miss The Mountains" because she misses her younger self, who felt pain, depression, happiness, sadness and even mania; but at least she felt.
Diana: [To Natalie] We tried to give you a normal life. (laughs) I realize now I have no clue what that is.
He's Back: When Gabe returns at the end of "Why Stay?/A Promise" behind Dan when Diana and Dan are about to embrace. Diana issues an "oh no" and turns Dan to face the (presumably) empty space where Gabe is, to which Dan yells a long "GODDAMMITTTT!"
Men Don't Cry: Averted. Dan hides his emotions from his family until it eventually tears them apart, until his wife leaves him and Natalie finds him sobbing alone in the dark and simply replies that "there will be light." Dan eventually agrees to see a therapist recommended by Dr Madden, after a long time of bottling up his emotions.
Only Sane Man: Well, there's the bipolar wife, the angsty teenage daughter, the dead-ghost son, the daughter's stoner boyfriend and the terrifying therapist. And then there's Dan.
While he's certainly more stable than the others, Dan is more troubled than he appears at first; a lot of his storyline throughout the musical involves him being forced to come to terms with his hopelessness about his wife's condition and his denial about the loss of his son.
Even Evil Has Standards: Many think of Gabe as a vicious and aggressive spirit/memory trying to spite his family into remembering him, but he stops following his mother for once when she leaves his father to help him.
One could argue that he was staying for his father for his whole appearance during the second act. He does reappear to Diana which tells us her treatment failed, but most of the song he sings during that appearance is directed at Dan, not Diana. And in his subsequent appearance, he is solely there for Dan.
Fanservice: During "I'm Alive", his rather... vigorous use of the tiered set definitely verges on pole dancing.
Natalie: And you play 'til it's perfect, you play 'til you ache. You play 'til the strings or your fingernails break! So you'll rock that recital and get into Yale, so you won't feel so sick, and you won't look so pale.
Henry: Our planet is poisoned, the oceans, the air. Around and beneath and above you.
Natalie: Um, Henry, that's true and I totally care.
Henry: I'm trying to tell you— I love you.
Power of Love: How Natalie helps the family come to terms with Diana's leaving at the end of the show, especially because of Henry, who is "like, number three on [her] list of problems" but her "favorite problem."