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.
Alternate Aesop Interpretation: The entire musical can be seen as a metaphor for the relations between the West and developing countries. The West (Chris) promises a better life to the developing countries (Kim and the Engineer) only to end up abandoning them (either willfully or after being forced to do so).
Fridge Horror: If you think Chris was going through hell before—having nightmares/grappling with his troubled marriage/struggling with guilt over inadvertently abandoning Kim—what's he going to be like now? Especially bad considering that he was making strides towards putting his life together.
It's not just him: consider what Tam's life is going to be like. His mother had to kill herself to force his father to take him and raise him in America, his stepmother flat-out doesn't want him, and he probably won't have any memory of his real mother at all. And even though things were bad for bui doi in Vietnam (case in point: Thuy trying to stab Tam for existing), America wasn't exactly free of Half-Breed Discrimination at the time either. This kid's most likely got a rough life ahead of him.
Adding to that, remember Ellen's line about she and Chris wanting kids of their own? If they do, this troper would bet anything that Tam ends up The Unfavorite in the eyes of one or both parents.
Hilarious in Hindsight: Jonathan Pryce originated the role of the Engineer, whose most memorable moment is probably the song "The American Dream". Twenty years later in G.I. Joe: The Rise of Cobra, Pryce played the American President, which is probably the ultimate American dream, fulfilled.
Just Here for Godzilla: The makers of the musical realised early on that the helicopter sequence was a huge selling point, so much so that the helicopter is the musical's logo on posters and promotional material.
Misaimed Marketing: This can probably be said about the decision to allow amateur theatre groups and high schools to stage their own versions of the musical.
Narm: The character of Chris. A good actor will usually be able to avert this but with a bad actor this will almost certainly happen.
Narm Charm: The musical can fall prey to this trope yet it's still utterly fantastic, watchable and will probably have you sobbing at the end. Chris is sometimes the most/only narmy one, especially during the song "Why God, Why?"; even its title sounds narmy.
It also depends on how well he can pull off that Big "NO!" at the end. Some actors are terrific, others have you stifling your laughter or even cringing.
The Scrappy: It's hard to find a fan of the show who doesn't hate Ellen, seeing her as the reason Chris and Kim can't reunite and why Chris isn't willing to take Tam (his and Kim's son) back to America with him.
There's also the timing problem with her marriage to Chris - he's been back Stateside 3 years, one of which was taken up by his Heroic BSOD ("spoke to no one for a year",) and they've been married 2 years. When exactly did they date?
There are many who view Ellen as their favorite character and others who see her as being as much of a victim of circumstance as Kim and Chris.
Adding to that, what little fanfic the show has portrays Ellen as treating Tam very well, sometimes even as her own son. Make of that what you will.
In the performance this troper saw, it was Ellen holding Tam and shielding him from seeing Kim's body, instead of the Engineer.