SWN is pure incline, and definitely my favourite by far of all the OSR "D&D but sci-fi" games out there that I know of. Also, both editions have a free version which only miss a couple of chapters on Mechs, AIs and AI player characters, tables to generate a planetary society's past and other such things that you can still have a campaign without (though they're useful if you happen to be into that particular thing). The paid version of Revised also has my real name in the Kickstarter backers list, so it's worth the money just for that.
Now, I’ll be mainly talking about the original edition, since I’m more familiar with that. For the DM, creating NPCs is much simpler than in 3E/Pathfinder, though that’s partially due to 3E/Pathfinder being overcomplicated and shit, but that's for a different conversation. If you just need a simple Level X Warrior/Expert/Psionic, there's a stock NPC list, you copy it from there, maybe change its weapon and a skill or two, BLAM, good to go. If you want to go the extra mile, you can roll on some tables for age, motivation, personality quirks and the like.
New alien monsters are also very easy to create: there are stock types like "vermin", "stalker", "multi-limbed horror" etc. with pre-set HP, AC, damage, etc. You then decide if it's insectile, reptilian, avian or whatever, and roll on (or pick from) a list of appropriate special abilities. These don't have hard rules and numbers for them, but you can interpret them freely and easily.
Note that creating a character in Revised is a bit fiddlier, though still nowhere near 3E with a bunch of splatbooks. There are fewer skills, but you also have "foci", which are kind of like feats, and Psionics is no longer "you have mana points and a few skills to spend them on", but a sort of skill tree where you have one or several basic psionic abilities and you can "upgrade" them. Say, as a Biopsionic your basic ability is to stabilise mortally wounded characters with a touch. You can later improve this to heal HP damage, but also give it bells and whistles like area of effect, working at range, healing poison and disease, etc. Still, there are quick character creation rules, and you should be able to write up an NPC in a matter of minutes.
Rules are easy to understand and use at the table, and the skill system is robust without needing a huge list of skills. The rules are also modular: if you don’t like this particular game"s take on poison or disease, you can just replace it with something else and it shouldn’t effect the rest of the game.
Now, GM's preparation. The game really fucking shines here, with the proviso that most of the tools are geared for sandbox adventuring. There’s a LOT of material to help you first set up a campaign (generate your star systems, planets, societies), and then to introduce new elements. A lot of this is down to the tag system: you roll or pick one or two tags for each world, like Badlands World and Minimal Contact. These in turn give you a short list of example friends, enemies, locations, adventure complications and important objects, which usually give you an immediate idea for an adventure or two.
Then there's the Factions system. This is something you’d use between sessions, though your players might also take the reins of a faction. A faction can be anything from a cult through a megacorporation to an entire planetary government. There are abstract "faction turns", say, between adventures, where the factions played out a sort of strategy board game without a board. They build and move assets like Guerilla Populace, Blockade Fleet, Psychic Assassins, Blackmail, Demagogue, Local Investments and Venture Capital. These can be used to "attack" each other, generating military, political and economic conflicts which can set the background for the campaign, or give the PCs adventure hooks.
A lot of this is geared at giving the GM something that he can do largely by following instructions, and which in turn give him adventure ideas. What all this DOESN'T do is give you a step-by-step guide to drawing a good dungeon and stocking it in an interesting sort of way, you'll still have to put your own work in that.
The original edition also has several (for-money) books that give you a lot of extra material if you want to take your campaign in a particular direction: espionage and black ops, space naval action or free traders.
If you're coming at this from a 3E/PF background, then I imagine the biggest hurdle would be the whole old-school mentality of "rulings not rules", sandboxing, and a reactive world where the PCs really affect the world around them in a way which, in turn, generates further adventures - though these are not so much an SWN thing but a general OSR one.
Hope that answered you question. If not, feel free to ask again, maybe more specifically, and I'll try to answer.
