I'm not enough of a game historian ("historical ludologist"?) to know when the trend started, but at some point players, GMs, and game designers got it in their heads that "balance" was the be-all-end-all of encounter design. It's why players scoff at tribes of kobolds, bemoan "quadratic wizards," and cry foul when you throw a full-grown dragon at a party of second-levels. It's why GMs consult table after table to figure out exactly how many gnolls their party can handle.
And it's really fucking stupid. Because it's boring.
There are a bunch of reasons why balancing every encounter makes for a boring game. First, it makes combat predictable. The players know that, barring ridiculously bad luck on the dice, they will survive every encounter so long as they don't act insanely reckless. Second, it forces the caster classes to blow their daily spells right out the gate. That's spike damage, my friends, you're supposed to save it for when it's really necessary. Third, it makes combat looong.
In a perfect world, you'd be able to get through a decent mini-adventure (or a good chapter of a longer adventure) in a 3-4 hour session. Let's say (optimistically, in my experience) that half of that time is taken up by roleplaying, bookkeeping, and problem-solving. That leaves only maybe 2 hours for combat. How long do your combats take?
A "balanced" combat against a party with decently sophisticated players easily takes an hour or more. So that means you have time for one balanced encounter. That's just fine! It just means that your other encounters will need to be unbalanced.
What do I mean by "unbalanced"?
The first way to make an unbalanced encounter is to bring in easy-to-kill NPCs. The four goblins in the tavern's kitchen. The nest of dire rats in the basement of the inn. The trio of skeleton guards at the mouth of the dungeon. These are all easy-to-pummel "tomato cans" you can throw at your PCs to break up the flow of the adventure. They make the PCs feel powerful ("Awesome, we beat them without getting hit!"), and they let the combat classes shine because the wizards won't want to waste any daily spells. It's the combat version of giving the party a locked door with a DC 10. It gives you the feeling that you're actually good at something, rather than slogging your way through an interminable skirmish with mid-tier enemies.
It also gives heft to the hard-fought win against the balanced encounter with the minotaur or the pair of leech-demons--or whatever. Where your barbarian was wading through kelp zombies before, now she has to trigger her RAAAGE to stand toe-to-toe with the Exalted Lich. It feels powerful, like your limited-use abilities really matter.
And that leads us to the other side of the unbalanced coin, the encounters that are virtually unwinnable. Say it with me: "Running away is A-OKAY!" Well, not really. You don't want to run away from everything, because that's for cowards. But every great hero (or party of heroes) runs away when it becomes obvious that they can't win. "Better part of valor" and all that. In the same way that an easy encounter makes the players feel big, an impossible one makes them feel small. You need a deft hand here, however, because there are some potential pitfalls.
The first is that the players, having been trained to think of encounters as "balanced," will stand and fight, taking it on faith that the GM wouldn't give them an "unbeatable" foe. Therefore, it's on the GM to make it clear, with statements like "IT IS OBVIOUS THAT THIS DEMON IS BEYOND YOUR ABILITIES TO DEFEAT," that a blow-for-blow battle is a recipe for a total party kill. This isn't railroading or metagaming. This is something that heroes face all the time, and they often know when it's going down. Hell, Gandalf straight-up tells the Fellowship to hoof it when he realizes there's a balrog on the way.
The second is that the solution shouldn't be as simple as "we run away." Turn it into a chase. Or allow them to sneak, trick, or fast-talk their way past the enemy, assuming such a thing is thematically appropriate. Pro-tip: it usually is.
Finally, you can always make the villain's victory condition something other than "killing the PCs." I'm a firm believer in applying the "fail forward" mechanic to every facet of RPGing, and that includes TPKs. Maybe the guards are *supposed* to beat the adventurers--duh, that's how they get to the dungeon. Or maybe the foe is only defeated through the sacrifice of one of the PCs, and the players now have a new mission: resurrecting or rescuing their lost compatriot. Turn the team's "failure" into a plot point or an adventure seed.
Anyone else have ideas on how to "mix up" your PCs' encounters? Anyone have experience with this way of doing things, either as a player or a GM?