Generic fluff is your typical inclusion. They are pretty much the same for fantasy and sci fi, and reappear in various forms. Here's a few of the top of my head. Maybe this could be a drinking game - take a shot each time you recognize one of these factions in a wargame.
Militant church faction. Crusaders, Inquisitors, hot-nuns-with-swords, etc.
Brutal, unsubtle, Orc-Barbarian-Norse bikie-gang faction. War cos... war is cool?
Usually fur, leather, and big axes/choppers.
Evil-for-its-own-sake. Chaos, whatever. Basically, we're evil cos... evil is cool?
I always wondered why "dark gifts" come with boils, horns, pus and mutations. I mean, not the most attractive recruiting method? I'd rather I became devastatingly attractive and muscular, whilst having the power to give all the boils and pus to my enemies.
Undead. Also see: exterminate-all-life robots. Because every bloody game must have zombies.
I never got why necromancers etc are so 'bad.' Because using rotting corpses to fight is more inhumane than conscripting teens/sending peoples loved ones to die?
Arrogant, advanced culture. Usually shooty. Usually those damn elves.
Evil megacorporation. The most plausible faction.
Empire. Usually modelled on the Roman Empire. Sometimes Russian/Soviet flavour.
Space 'Murica. Freedom! Bestest faction! Usually comes in red, white or blue livery.
Devour-all-things. Lots of fangs and claws. Hivemind/Queen likely.
...anyway, you get the point. There's very little new under the sun, and most are variations on a similar theme. So this begs the question - if a games fluff and setting is identical to half a dozen others, why would I want to play it?
Unless, of course, all you are trying to do it show you can use anothers' setting using your game engine. In which case you should make the parallels as un-subtle as possible as you try to piggyback on the success of others. Which sounds a bit lacking in nobility, but is certainly a sensible option.
The Magic of Shiny ThingsDo you really think a written page or two of "setting" will make someone fall in love with said setting? Most top sci fi and fantasy authors get 500+ pages and can't do it.
I've been reading a lot of RPG settings, and to be frank, the standard of writing is pretty awful. People inventive enough to make great games do not always have writing talent (and vice versa).
But most successful settings (almost irrespective of literacy merit) invariably have art with high production values or an attached miniature line. Shiny stuff, in other words. I really can't think of any settings which can successfully engage people with a MS Word document with a few typed pages and some generic clip art. Certainly not one that uses the same generic tropes that half a dozen other settings do.
It's naive to assume anyone will give you fluff more than a passing glance unless it has the "shiny" factor. Just describing your fantasy race, without even pictures... ...no, it's not going to make your setting the Next Big Thing. In all likelihood, they never even glanced at the page.
Focussed SettingsThis is a setting focussed on a small area or portion of the game world.
This does not mean making the game universe a small one, or making the game universe "rigid" - I call that a constricted setting - like 40K has become.
Originally, 40K was an infinite universe, full of any number of weird alien races - of which the official races - space marines, space dwarves, space elves, space imperials and space orcs - were only a small part. Now it restricted to pretty much the Gothic Sector, with only the official races who are locked in eternal war with each other, kinda limiting your options into a kind of deathmatch situation in a limited arena. Storytelling options are "x fights y in z" or "a fights b in z." Not only that, but the timeline is tightly filled in. All the important events are documented - you kinda have to play their events, rather than being free to invent your own.
A focussed setting is different. It focusses on one aspect or area of the game world, but allows flexibility, creativity and latitude outside of that. Infinity does that - whilst it "zooms in" to local conflicts, whilst allowing that it's a big universe out there - they already have two alien factions, and have the option of adding more at any time. It's focussed on human space - and only a few specific planets at that - but allows itself a wide open galaxy with the potential for future expansion.
A focussed setting allows a smaller game/developer to do a good job, by narrowing the game designers/writers efforts. It allows for detail without bloated, unwieldy exposition. Frostgrave was a great example of this - and in fact what inspired this post. Its "explore a frozen city, unearth magical treasures" theme was both a clever appeal to both piggyback nostalgia for another, previously popular game "explore a ruined city, unearth magical warpstone" and also to focus the writers' efforts and creativity. It doesn't restrict me - I can use the games and rules to play the game outside of the frozen city of Frostgrave - heck I can use it for my own fantasy settings if I want. But by focussing on a single aspect of the setting, and providing plenty of shiny (both nice artwork and a small miniatures line) I'm inspired to play in the setting - even though the actual "factions" are rather bland, generic human mages with generic human fantasy mercenary troops. However the focus on mages as the main character is (apart from Warmachine) relatively unique.
Remember - a focussed setting merely means the rules writer concentrates on describing a small area of the game universe, whilst leaving plenty of "wriggle room" outside of that. It does not mean a constricted universe, with "set" factions, limited army building options, and a rigid, overly-detailed timeline. It just allows the writer to focus their efforts to give a better "feel" - and minimise bloated fluff.