In my local D&D game, I ran what amounts to the same encounter two weeks in a row.. Both were level 1 encounters, 500 xp, and consisted of 7 monsters. 1 Hobgoblin Soldier, 4 Hobgoblin Grunts (minions), and 2 Goblin Sharpshooters. The first week, the fight took place in a forest as part of an attempted ambush. In the second week, the fight took place in the Rivenroar catacombs. The ambush in the forest was a pretty exciting fight. The fight in the catacombs, well I somehow knew from the moment I set it up that it wasn't going to be that fun. For me at least, as the DM, it wasn't.
So what was the difference?
Location. Last week the battle took place out in a forest clearing along a faint path leading from the town of Brindol to Rivenroar. There was plenty of cover and difficult terrain for both sides to deal with. The two Goblin Sharpshooters were perfectly in their element, well hidden and able to snipe away at the PCs. The area made it an interesting battle.
This week was different. The room was largely wide open. The minions couldn't take advantage of cover to help keep themselves alive long enough to do even a little damage. The goblin sharpshooters were easy meat, unable to hide well and take advantage of their special abilities. The lone Hobgoblin soldier, who used terrain to guard his flank and hold off the PCs from finding or attacking one of the sharpshooters, was quickly surrounded and demolished.
The PCs had a great time in this fight, don't get me wrong. They walked all over this fight with barely a few scratches. Even if I'd rolled better (I rolled quite poorly), I don't feel they would have been all that seriously challenged. But it wasn't much fun for me and I don't mean that because the PCs had a pretty easy time. There was no real kick to the fight. No spice. Although my monsters had special abilities, they didn't have any chance to really use them because of the terrain. Could I have done things different? Yeah, it's certainly possible. However I'm not sure that in the end it would have made a lot of difference.
I knew before, in the back of my mind, that terrain can really impact a fight. Whenever I'd designed encounters before I'd done my best to make the terrain interesting, unique, or something to be wary of tactically. I feel I did a decent job at that. However, I'd never had quite such a vivid demonstration of how much a terrain can make or break a fight as I did tonight.