@Zilfrel: Classic stuff to throw in. Traps and swarms definitely adjust the tactics of the party, I agree. What sort of traps do you feel work against high level parties?
@TimD: I agree, working up to it is normally how I prefer to have things happen. However, the abrupt end of our other campaign, where they had earned their way to higher levels, left many of them still wanting to try high level game play.
I love the idea of interaction between lower power adventurers and the higher level PCs that you've mentioned. That sounds like an excellent dynamic to have the players be the ones with the power giving out wisdom. Lair building also is intriguing to many of my players, I'll definitely include that.
You mentioned "Putting the PCs in a position where a single, painful choice is the answer to the continuation or destruction of a people", which sounds like a very memorable situation for the players. What was the painful choice they had to make? What was the result for the players?
@Human Diversion: Were the mounted trolls with drow backup an encounter the players could prepare for, or an ambush? I'm assuming it was out in the open where the trolls could charge? What was the end result of the encounter?
@Rarednaw: Excellent stuff to work with, thank you! In particular, I'm curious about the story of the endangered species that is Balor. Were there any consequences for removing that "species" from existence? Or did the players attempt to save some of them? What happened there?
@Bacon666: Subtle disguise shift sounds like an excellent way to tweak with some of the more well read members of my group, thank you for the idea. What was the result of the dragon fight? A narrow victory? You mentioned that the goal is to make encounters hard pressed, what makes you as a player feel pressured at high level? Being low on spells, using up consumables, almost dying, etc?
@aceDiamond: Brilliant. I greatly enjoy when my players come up with some unexpected plans, though I suspect I'd have been just as alarmed as your GM. Was finding a unique vulnerability the most memorable part for you, or just the fact that you found a unique use for a lower level spell against a higher level foe?
@everybody: Thank you again for your responses. It's definitely helping me expand upon the ideas I already have in place. Keep 'em coming if you've got 'em!