In previous editions, most DMs made thinly veiled attempts at pretending they weren't deciding things in advance. So, you'd create a dungeon, populate it with monsters who were evil, protective of their home, violent, and quick to provoke. Then you'd seed a plot hook to give the PCs a reason to go down there...possible a reason to want to kill off the monsters.
And sit back and watch the fireworks(mostly consisting of a series of combats). All the while safe in the knowledge that you didn't PLAN there to be combats...the PCs could certainly come up with alternate ways to get the macguffin out of the chest underneath the Goblin King's bed in the heavily guarded Goblin outpost. But, the result was all but certain in advance.
Having used the same scenarios with multiple play groups, I have to disagree. The result was not all but certain in advance.
To take two simple cases from Keep on the Borderlands (and, if you don't want spoilers don't read on), which I have run in Holmes Basic, 1e, 2e, and 3.x:
[spolier]In KotB, there is an evil temple in one of the caves. That temple contains a chamber where acolytes can be found. Depending upon the approach of the players, I have had this scenario turn into a general melee, a chase, a running battle, and a religious debate. Even when the encounter turned into a combat, there can be tremendous variations in how that combat occurs simply because it isn't predetermined.
Likewise, in one cave there is an owlbear. The odds are pretty good that this is going to be a "combat encounter", but what type of combat it is depends very much on the group. I've even run this module where the owlbear was used by players to take out another monster. Because the surrounding humanoids know that the owlbear is there, the PCs can learn this too.....some groups wait for it to leave the cave to hunt, choosing to avoid the encounter altogether. Others devise ambushes or traps to lead the critter into, knowing that it is too tough to take on in a straight fight.
When I ran it in 3.x, the party stumbled into the owlbear while fleeing the grey oozes in a related cave, which created a very different encounter than would have occurred had I planned a "scene" or a "combat encounter" beforehand.[/spoiler]
The more you envision what will happen beforehand, the more you limit what can actually happen in game play.
RC