When y'all are browsing through a game book in a store, what makes you decide to buy the book?
Assume that it is a system you have not tried, you have sufficient time to browse through a book, and the book is open (not shrinkwrapped or anything).
For me, with a science fiction game, I'm almost always looking for something with ship/unit design rules and with good tech research rules. After that, playability. Background is good, and can be vital to keeping my interest, but if ship design and technology isn't done well, I lose interest quick.
For example, Traveller had an excellent background, and good ship design rules, but was cumbersome to play on any scale beyond ship-to-ship and the technology levels were pretty basic. Full Thrust is simple to play, and has a basic background, but the ship design was way too simplistic and the tech stuff was rudimentary. Still, it was fun to fight out simple battles in that system, but the simplistic ship design stuff turned me off.
Really, Starfire had the best combination of medium to large scale playability, ship design, and technology of any game that I had experience with, which was why I kept coming back to it. Some games were more playable, others had more complex (and probably realistic) ship design stuff, but none had the combination of things that Starfire had.
Of course, I'm probably atypical, as I usually will evaluate a new system as to how it will help me develop my own background. If that system comes with a background, I'll usually look at it more to see if I like it and can use the concepts more than anything else.
Kurt