I'm running a D&D game with some friends right now, and they've made it up to 12th/13th level. One player told me he doesn't care for higher-level games of D&D, and claimed that at those power levels, each encounter is either an absolute victory or an absolute failure; you're either well-prepared to handle that challenge rating of an encounter or you're not. I thought about that a little bit, and considered how I create encounters for my higher level games.
I don't believe that, at higher levels, an encounter is any more an absolute victory or absolute failure any more than at lower levels. But then again, many of my encoutners aren't the traditional D&D kick-in-the-door, deal-with-the-monster encounters. Here are a few things I do to mix up my encounters, to make even slightly lower-level creatures a higher-level threat:
1: I'll play with the battlefield. A dungeon encounter may include traps along with the monster, blockades, or nuisances that can slowly deplete characters' resources while in a combat. for outdoor encoutners, I'll use buildings, foliage, and geographical features to augment the creatures' abilities.
I had one encounter that spanned the interior and exterior of a run-down inn, which included, if I remember correctly, 2 ogres, a goblin sorceror. The sorceror hassled PC's left in the inn, used cover, and threw spells outside to PC's combatting the bigger guys. The ogres were steamrollers, destroying any cover the PC's were using.
2: Social combat always makes things interesting. Obvioulsy, not every encoutner has to be combat-oriented. We've had one session that was just the players negotiating with a dragon NPC, and making it out safely (with tons of information relating to storyline in the campaign). Some levels of social interaction tend to spice up encounters that are even 'destined" to become combat encounters. They keep all the players' attention, and still give the opportunity to use characters' skills and other resources.
3: The stakes get bigger. Lets face it: in D&D, fear of character death isn't a terribly huge motivation. There are too many opportunities for Raise Dead-style spells. Once the players begin to care abou the campaign world, though, any threats to the world itself become a bigger motivator for the game. The trick is, getting the players to care.
That's always the tricky part. I'm lucky that, as of right now, I have players that do have their characters care about their campaign world. When they don't, then its back to some old-school GM tricks: they probably care more about their treasure. So its time to undertake quests to remove curses from their magic items, etc.
None of this is new or revolutionary. Everybody who has played or GM'ed in a RPG has figured alot of these techniques out. I find it interesting, though, how much you have to remind yourself about them for D&D. Its like they're easy to forget, once you start mapping a dungeon or rolling on the treasure tables. The trick is to not let them go, I reckon.