General Overview
Sci-Fi RPG's have always had a hard time spreading in comparison to fantasy. They lack the 800lb Gorilla that is Dungeons and Dragons (despite repeated attempts by TSR/WotC/Hasbro) and so the title of king of space sci-fi RPGs has been a lot more fluid with flavours of Traveller (with its infamous death-during-character-creation) generally on top. Mothership (and its more recent copycats) is a game that quietly stakes a new claim on the throne and has a lot of merit to do so. Mothership leans into the non-descript "Blue Collar" sci-fi epitomized by Alien at its best and Space Truckers at its worst. Knowing most game groups I therefore suspect "Red Dwarf" will be the replacement of choice for the role Holy Grail quotes play in a fantasy RPG.
A Mechanic that works really well
This isn't a specific mechanic (look at me breaking my own rules) but rather a keen understanding of the point of RPGs. Something I have long harped on and so its great to see a stellar example of what that type of logic means.
The Warden (the GM/DM/Referee) book has a full break down on how to craft the type of adventures Mothership is meant for. Among the well designed walkthrough is the concept of the 3 S's: Save, Survive, Solve and the concept that it shouldn't be possible to do all three. "Save people, Survive yourself, Solve the mystery/problem". An adventure design version of the "Good, Cheap, Quick" dials for producing anything.
This guidelines on how to actually USE the system at the table to make the kinds of adventures its meant for has (I strongly suspect) been key to its popularity. The book assumes you are a newbie and teaches you what you need to know to be competent at making fun adventures. Sure it won't make you an expert, but they put the work in to drastically reduce the time it takes to becomes competent at running these types of stories. That isn't easy for a writer to do and it isn't as much fun as working on advanced concepts. Case in point, here is how I addressed that in NGR on the first page:
"This text is written assuming you already know the basics of how to play RPG's, because I have zero interest in rehashing those talking points for some theoretical yet non-existent reader."
That is me refusing to draw the rest of the fucking owl while Mothership went ahead with a full photorealistic picture of an owl.
A detail that lifts above its weight
The easy answer would be to point to the table for rolling jacket patches and what the entries say about the implied setting, but that is a big table with a lot of effort put into it. It lifts, but its also a big chunk of real estate.
No, the rule that lifts above its weight is the "High Score" stat, which is just how many sessions you have survived. This combined with the text telling you the average high score is 4 (which think about it, how could you possibly know that?) does an amazing job in few words of setting the lethality. Half of all characters never make it to 5 sessions. You have just set the pacing of the game in a very visceral way while not setting a hard limit. Genius.
Constructive Feedback
Mothership is great for shorter games and that is a real strength, it is perfect for conventions and for gamers in the professional/child rearing stages of their life. This means it needs to be kept simple and quick to both pick up and pick up again when your gaming group hasn't met in 4 months because life kept happening and getting quorum was impossible. Hell the Omega session deserves an honourable mention in terms of good ideas in the book.
That makes it a bit hard for me to give constructive feedback that fits in what Mothership seems designed to do. There are nitpicky minor things I could suggest, but based on the purpose of the system I don't think they are really relevant.
So in the vein of this review already breaking the rules I had set for myself, I will suggest that what might be helpful is a version of the venerable Appendix N. While I know that books and videogames would be a lot of the actual inspiration, I'd suggest continuing to lean towards the needs of time crunched mid-life players and just put a "Watch List" of movies to get the aesthetic. Not just good ones like Alien and Aliens, but shitty ones like Creature (1985) and Saturn 3 (1980). In fact I think shitty movies are MORE important in terms of setting the bar low for new Wardens. Don't make them think their games have to match the perfection of Alien, let them know stumbling along with a shitty B-movie equivalent is still a win and a good time.
Failing that as a suggestion, I guess you could rename the life form scanner to the H.R. Geiger counter.