The Mothership RPG is a mutant deriving primarily from a mashup of WFRP 2e and old-school D&D with bits and bobs drawn from a half-dozen other systems, with a strong presentation that focuses on conveying information clearly using flowcharts, illustrations and diagrams. There are classes but they mostly determine your starting stats rather than your skill progression, and the core mechanics for doing things are built off of skills rather than your class or levels.
It's meant to primarily run space horror games, but I think if you have access to Traveller and Stars Without Number, you could very easily use it to run a space opera game by plugging in material from those games, and there's just enough alignment around certain features of how space travel works in Mothership that it should be fairly easy.
The two most convoluted systems are the panic & stress system, and the ship-building rubric. The former is IMHO just slightly more complicated than it needs to be, and it lacks a graphic flowchart because stress and panic checks show up in the middle of other processes instead of being a separate subsystem. The ship-building diagram helps, but isn't as clearly laid out as many of the other flow-charts, and some of the phrasing requires you to read it a few times to understand exactly what's going on.
That said, if you're looking for a relatively light d100 relatively-hard science fiction system, Mothership is pretty good. It passes the essential test (I once heard it called the "Chubb test" after the last name of the RPG designer who first proposed it as a test of the quality of RPG products) of making me want to create a character or run a game after reading it. The flowcharts, diagrams and other graphical elements really help explain how the game works concisely and effectively, and I'd love to see more games that incorporated these kinds of elements to explain their subsystems.
The main change I would make to the rules-as-written would be to change the core mechanic to stat +d% to beat a target number of 101, since this is intellectually an easier mathematical operation for most people to perform than rolling under a variable target number. The second change would probably be to slow down levelling slightly - characters get 10 XP per session survived (in a system where PCs level up with tens of XP instead of thousands), which implies to me a short campaign built around a single central mystery or horrific experience with a moderately-high fatality rate. The third would be to simplify the panic and stress rules in a couple of ways. First, I would make "Resolve" simply work as a bonus on your stress checks (where you are trying to roll over your current stress). Second, I would organise the panic effect chart into three bands - 2-10 (mostly positive), 10-20 (mostly negative), 20+ (cripplingly negative), pile a list of options under each one, and let the referee or PCs choose from the list each time for simplicity's sake.
Overall, my impression is a positive one. I'd love to see a follow-up "Referee Guide" that focused more on fleshing out the implicit setting and constructing mysteries / horrific situations for the players to survive. The Mothership Player's Survival Guide is a 44-page PWYW pdf or is available directly from the author as a $12.00 printed book.