NPCs are the bread and butter of a DM’s toolkit; unless you’re running an adventure deep in the wilderness, your characters are going to run in to other people. And if they have access to a settlement of any decent size, they may easily meet lots of people. It’s possible to hand-wave this so they interact with nameless merchants and get rumors from faceless street urchins, but in a lot of cases that could lessen the game. So lots of DMs put work into coming up with ways to make quick NPCs, and thought I might add such a method to my Toolbox here.
Given my assumptions, almost everyone my characters meet with be Level 1 Commoners with average stats. The few craftsmen may be Experts, men-at-arms will be Warriors, and the rare witch or holy man will be an Adept. Rulers and high society will be made up of mostly the same, with the top few actually being classed as Aristocrats (just because you’re high-born doesn’t mean you necessarily take that path). In short, most random NPCs are probably going to be Commoners or Warriors.
When I’m actually statting an NPC, I like to use the Basic Array (13, 12, 11, 10, 9, 8), but it’s probably best to assume that most people will have 10s across the board (it’s Average for a reason). Conveniently, Commoners and Warriors both get 2+Int Skills, and have essentially the same class skills: Climb, Craft, Handle Animal, Profession, Ride, and Swim — Warriors add Intimidate and Commoners add Perception. Pick two skills to get a +4 on and everything else is a +0 and you’re done.
Warriors are proficient with all weapons and armors, 5hp, and have a +1 BAB and +2 Fort. Commoners are proficient with a single weapon and no armor, 3hp, and get no other bonuses. You can usually ignore things like Feats and Traits for random NPCs (if they become important enough to care, you may just fully stat them between sessions).
If necessary, add a +1 bonus to the appropriate stats if you’re creating a non-human NPC (so elves get +1 in DEx and Dex-based skills, and +1 in Int an extra Skill).
What’s In A Name?
The biggest consideration (since stats are fairly straight-forward) is the character’s name; if you’re doing it off-the-cuff you’re likely to end up with something silly-sounding or “Bob.” The best thing to do is generate a couple dozen names for Males, Females, and Surnames, and just mix-and-match as necessary (I could easily see a random table for putting together names, and you could vary the frequency of certain names if you think “Tomen” is a common name for halflings in your world). I generally choose a style for each of my main races: dwarves have Norse-based names, halflings have Gaelic-based ones, and so on.
On Assumptions
An interesting consequence to these “average NPCs” is the fact that most people will easily die to a single sword thrust (the expected result of a Short Sword is 3 damage, 4 for a Long Sword; a Greatsword will fell even trained men-at-arms in one swing), and a creature that has a +2 to damage is essentially guaranteed to kill them if it hits. This makes Orcs (with a +2 average STR mod) much more frightening to regular folks.
Jack's Toolbox 
A few thoughts here very similar to my own, http://shortymonster.wordpress.com/2012/07/16/cutting-corners-not-quality/
Although I have the advantage of not usually dealing with fantasy races in games that I run.
Yeah, I read that, but it didn’t really address stats (or I’m unfamiliar enough with UnMet that I couldn’t recognize stats when you mentioned them). As for fantasy races I can’t find the blog post now to link to it, but I’m actively trying to move my D&D world away from “The Star Wars Cantina” where every race is everywhere, so it doesn’t matter, and more to most people having little experience, if any, with other races — and so I’ll really only need to deal with one species of random NPC at a time.
Yeah, the best link I could find was to the character sheet, and that was mainly for illustrative purposes. But think 2 points across the board for human average, then pick one or two that would be different and just make a note of those. It does get a bit of a pain sometimes using a mostly unknown system for comparisons, but it’s a game I run a lot, so a big slice of GMing experience comes from it.
I’m definitely in favor of writing what you know — at the same time, since I’ve started blogging I’ve decided I need to broaden my horizons and at least become conversational in other systems. I keep meaning, in particular, to read up on the OSR retro-clones, since a lot of what I’m doing here dovetails or overlaps with that community.
To your point, though, “2 points across the board” doesn’t mean anything to me in the same way that “10 in all attributes” doesn’t mean anything to someone who isn’t familiar with how D&D works.