In his blog post TROUBLES & TERRAIN , The_Jester brings up the importance of terrain in 4e and the added difficulty it can cause players without any added reward. He suggests that if the terrain makes the players chances of winning that much harder that it should be reflected in the Encounter Level and thus the experience reward. I agree with him but I also think DM's could benefit from thinking about role the terrain is to play in the encounter with as much forethought as creature selection. Is it Hindering, Neutral or Helpful?
In cases where terrain increases the actual difficultly of the encounter, it's almost always a result of terrain being used against the players and the monsters or NPCs having some sort of mechanic of circumventing penalties or maximizing bonuses. Too often, in my opinion, do DMs use terrain in this manner almost exclusively. The terrain often manifests as a global environmental effect such as the classic/cliche "icy floor." (A perfect example of which appears in the videos of the Robot Chicken writers playing D&D on the main WotC D&D site.) These rooms tend to slow down play for everyone while particularly punishing mobility based characters over ranged attack characters who might not be effected much at all. Rather than make the encounter more interesting, they increase player frustration. If a player rolled up a mobility based striker then that's probably what they hoped to play. Once they start spending move actions to get back on their feet every other round, they are bound to feel a bit cheated. For any character class, making a successful roll just to remain standing and use your regular at-will power isn't that heroic when you have to do it every round.
If DMs layout terrain to the players disadvantage, I would recommend that they still include normal terrain and let the players make the choice of whether they will be funneled surefooted into a well guarded choke point or make a dangerous dash across the lava pits to flank the enemy. Make willingly subjecting one's character to terrain penalties a daring yet calculated risk. Or at least something that that can be slogged through once and then overcome. Terrain that causes damage which can be avoided on a successful skill roll might grant XP per square successfully crossed, encouraging feats of daring-do. Give the players a sense the terrain can be "beaten" and not just waited out until the end of the encounter like it's a flu.
Not all encounters need be ambushes at the location of the NPC's choosing. Hasty meeting engagements may have both spots of Helpful terrain and Hindering terrain more or less equally exploitable by both sides. Slavish desire to balance the battlefield isn't necessary, just place terrain in a sensible manner without thinking about the tactical implications. Once the terrain is position, imagine how the battle would play out differently if players and NPC's started on opposite side. If the neither position has a significantly greater likelihood of TPK, you've probably done alright. For outdoor encounters, you might decide the starting map edge for each side with a random roll. Standard experience rewards are the order of the day here.
Sometimes it's fun to sit behind the castle walls and let the other guy worry about how he's going to get to you. One of my favorite encounters as a player was defending a fortified farmstead from a goblin horde in the D&D module Night's Dark Terror. You should throw these sort of encounters the players way every so often to keep their moral up. It doesn't have to be a complete calk-walk but even so, an occasional turkey shoot is good for the soul and lets the players run their heroes as the success prone demigods they imagine them to be.
However! If the player are to be rewarded for starting at the bottom of the hill, they shouldn't expect a full share of the XPs for starting at the top. Consider reducing the XP reward by an EL or two as appropriate.
"Not tho' the soldier knew, someone had blunder'd… Charging an army, while all the world wonder'd."
Adjusting player XP rewards based on terrain is fair practice when the DM is picking the place but what about the times the players decide when and where to fight? Should players who foolishly insist on reckless and ill-prepared frontal assaults on disadvantaged ground when other options are available to them still receive a XP reward bonus? Is it punishing good behavior to reduce the XP reward when players use creativity and sound tactics to trap their enemies at the bottom of a dead-end box canyon? I can see both sides of the argument whether to adjust the XP or not. I am leaning slightly toward the thinking that adjustment is still appropriate; the players should be rewarded for surviving the experience, no matter what choices lead to the encounter. Dead characters are punishment enough. A closer reading of the rules on XP rewards is in order here. I'm nearly on the fence on this one and could be persuaded to another position.
So, tell me DMs, what are YOUR thoughts?