YMMV / Project Hail Mary

• Big-Lipped Alligator Moment: One of Grace's flashbacks has Stratt engaging in a court battle of intellectual piracy, since Stratt had pretty much every bit of software and other IP ever made uploaded to the Hail Mary's computers, which the IP holders maintain is illegal. It's one of the shortest flashbacks, Grace doesn't play any role in it, and some people feel like it's there just to serve a Take That! against overzealous copyright laws.
  • On the other hand, this establishes both the backstory behind Grace's access to every type of software and every piece of information he needs, and serves as the only indication of what happens when Stratt's uncompromising nature steps of the toes of moneyed interests (she still wins).
• Fanfic Fuel: What if one or both of Grace's crewmates, or any of Rocky's, had survived the trip to Tau Ceti? What if Grace himself hadn't? What if DuBois or Shapiro had gone on the mission as planned? There are a lot of potential ways the story could've unfolded had the circumstances been slightly different. And that's not even going into what might have happened on Earth after the Hail Mary departed...
• Genius Bonus: When Rocky hands over the piece of iron and insists it is "twenty-six", we are first confused like Ryland is due to him not grasping that Rocky is communicating the atomic number (not the weight in some weird Eridian unit). It becomes clear only later that there is a second level to this - Rocky seems to only be aware of the atomic number (i.e. the number of protons and electrons) but not the atomic weight (i.e. the - average - number of protons and neutrons) so he is not aware of isotopes. And why would he, his species has no knowledge about nuclear physics and for the kind of chemistry Eridians are doing, all you need is the Atomic number anyway.
• Inferred Holocaust: The astrophage is affecting all stars within an ever-growing radius of Tau Ceti. It's noted that, since Earth and Erid, being relatively close, both have intelligent life, there are pretty strong odds that the same is true of other nearby systems. Without the capacity for space travel, any other civilizations would likely be doomed.
• Moral Event Horizon: One might reasonably feel that Stratt crosses it when she forces Grace onto the Hail Mary, a Suicide Mission, against his will, imprisoning him for the remainder of his time on Earth and lying to her superiors that he had volunteered because she knew they would object if they knew the truth. Granted, the fate of humanity depended on the mission's success, but it's explicitly noted that there was at least one other qualified candidate who actively wanted to go on the mission, but she forced him into it anyway because he was the more ideal candidate.