I've messed around with a few things in my latest round of encounter design for my Greyhawk game. I've also drawn on 4e's ease of mathematical use to help implement these things. Here's an example:
In my Greyhawk game today, the characters fought against a priest of elemental evil in a chapel. A shell of elemental energy surrounded the priest and a statue in the center of the room. The shell was composed of cold and earth magic, spawned by two glowing orbs of energy that were on opposite ends of the room.
The PCs had all sorts of fun pushing the wights and ghouls that guarded the chapel into the energy shell. It froze and battered the undead. That is, until a ghoul died in the energy shell. Then the shell grew dramatically, catching the PCs in it. Their safe position at the room's edge wasn't so safe anymore.
In addition, a successful Arcana check told them that if 2 more creatures died in the shell, or if the priest died within it, the energy spheres would unleash a pulse of elemental energy. That was a bad thing, though it wasn't exactly clear how it was bad.
So, the fight turned into an attempt to kill the undead while keeping them out of the shell. The high priest started provoking opportunity attacks and fighting recklessly. It had 2 hp when its next turn came around, so it leaped into the shell and died.
There's nothing special about all that stuff (well, aside from proving an enjoyable fight to run), but something that I found neat happened behind the screen.
I didn't script any of that stuff out. I had some general notes that the energy field would do wacky stuff, like grow and move, but I made a point of not writing down exactly how that would happen. Instead, during the session I found a dramatically appropriate moment and found a reason for the energy field to expand.
DMs who are any good at their hobby know that improvisation is a big key to keeping the game interesting. The players do unexpected stuff, but so should the DM. I've had a lot of fun the past month or two stopping two steps short of fleshing out an area, instead noting the general *stuff* that makes the place interesting and leaving detail to the PCs. I figure if they can spring stuff on me to ruin the PCs' plans, I can spring stuff on them to ruin theirs.
The key to why this has been fun, as opposed to arbitrary, is that I've never gone back on something I've already said or used in my description. I think that lets the players make informed decisions.
This advice is, I'm sure, no news to anyone who has run a lot of OD&D or AD&D. The OD&D sessions I ran last year were an extended exercise in that method. What I like about 4e, and one of the things that I'm most happy with the design, is that the game has transparent, easily understood math that makes this stuff easier than ever. I have a ton of room to improvise as a DM, while the players get lots of crunchy bits to play with.