I think my two main gripes were:
the entire thing is very casually american-chauvinist, and then human-chauvinist
every sci-fi trope there has been done better in other books. most of them have been done better in Peter Watts' Blindsight
It is solid sci-fi but is also like Scifi For People Who Don't Like Scifi. here is a bulleted list of things that pissed me off, most of them relating to the above two points, most of them notes taken while I read the book
the book was way more bootlicky in its depictions of the military but the movie was also pretty uncritically praising them. verbatim quote from book: "I have the US army with me. And it's a damn good army."
the book's narrator remarks offhandedly that for some funny reason the PLA (chinese) Navy is named that way. does not know or care to figure out why
they had a character say "don't worry, I'm one of the good Canadians, not one of the ones who hates America"
marvel quips throughout. the earth's supreme cold rational-above-all-else science lady still finds time to say "Yeah. That's a thing now." and other assorted moronic tidbits
the narrator in the book is an insufferable Redditor that Gosling did a good point of toning down. might be how his character was written but it felt like Weir really thought he made a funny likeable protagonist. all the book dialogue is a vehicle for exposition.
for the amount it's lauded, all the Hard Scifi Physics is high school physics at best. there is no calculus, linear algebra, or vector math at all
the high school biology level of the aliens (wow! guys, they're cells! made of water! and get this - they reproduce normally!) is gone into detail to an insufferable degree, until reality meets the wall of scifi and Weir would actually have to make an interesting call about why they can survive in the sun or in deep space or whatever. then it's "oh, I can't tell, we never figured it out, we don't have time for this right now." we aren't even given any interesting hints as to how the MOST INTERESTING SCI-FI part of the space microbes' biology might work, but are treated to breathlessly explained paragraphs of normal cell biology facts.
but really the thing that pissed me off, above all else, is how his communication with the alien was portrayed. the alien was clearly sapient, their conversation was mutually intelligible with new words being added constantly, and yet.
the alien's speech is only translated into this stupid fucking pidgin english!
that reduces the alien (in the book and movie's portrayal, a blue-collar character who's willing to sacrifice his life for another alien) to little more than a pet dog. and it's not even funny or interesting pidgin english. it's the most unimaginative "normal sentence with prepositions taken out" shit ever
and it is solely there to change the portrayal of another intelligent, sapient creature, fully capable of interstellar travel, to a goofy sidekick who backs up the Good Old American Ingenuity of the main character. only at the end of the book is the alien allowed to speak proper English.
if you really liked Project Hail Mary and you haven't gotten into that much sci-fi, sorry to shit on it but there's a bunch of cooler stuff out there that revisits these themes and tropes in a much more competent, less /r/hfy self-masturbatory drivel. you should read Blindsight if you like the idea of an autist being sent out into deep space to find something scary and try to save humanity, or Embassytown if you are interested in human/alien speech barriers. if you already like and consume a lot of scifi and also really liked Project Hail Mary, I think you probably have a dent in your head that needs ironing out
