Because of the importance of movement, terrain, and a general increase in the amount of tactical options 4e encounters have more potential than 3e encounters; defenders can actually do their job, leaders have a bigger toolbox and more available actions per turn, controllers have a more nuanced job to do, and strikers (as well as everyone else) genuinely need the support of the rest of the party. This is inherently more exciting that walking up to a single enemy and standing toe to toe with them, round after round, unloading full attack actions (ok, perhaps "exciting" is too subjective a word, but it's certainly more dynamic). But enough with edition comparisons. I merely want to illustrate that, while 4e combat can be run in a simplistic way like this, it can also be much more interesting, and that obviously starts with encounter design.
One of the most commonly touted pieces of advice that I hear about 4e combat (and to some extent with combat in any edition) is that it all starts with terrain. Fighting on a flat, featureless plain or in a 5x8 "box" of a dungeon room is neither evocative nor does it offer anything for the combatants to interact with. At the very least a DM should provide a couple of pillars, overturned tables, or bushes to be utilized as cover and perhaps some sections of undergrowth or rubble to serve as difficult terrain. These things are easy enough to add to a map, and while they're pretty simple features they still present a spatial goal to both the monsters and PCs; occupy terrain that provides an advantage, and try to hinder your enemy with terrain that provides a disadvantage. If the PCs aren't taking the bait, as a DM you should make the monsters set an example! One important piece of advice is don't overuse difficult terrain! If most of a map is a field of rubble, encounter after encounter, it ceases to be interesting and becomes merely annoying.
Terrain can of course be much more complex than the example stated above, and may even consist of things like terrain "powers" (see the DMG2) or hazards. Pools of acid, pits, cliffs, or fire pits are great when combined with forced movement because it encourages PCs to interact with their environment in a really obvious way. Make sure the monsters have some form of forced movement if you feature such hazards, if for no other reason than to ensure that the PCs need to be on the defensive. If a monster has no native forced movement powers, it's easy enough to add one! Just tack a push onto a melee basic attack and call it a different standard action. Don't go overboard with altering monsters like this, though; if all of your monsters conveniently have push attacks the technique will grow stale, and it will challenge verisimilitude.
Since most dungeons commonly involve the PCs invading the territory of their enemies, it makes sense that many of those enemies (especially the intelligent ones) will know how to best take advantage of the local terrain, and may even have modified it to suit their needs (including things like setting traps and ambush points). For example, artillery should generally have cover to hide behind so that PCs have to close the distance to them before they can attack without penalty. Additionally, it should be hard and/or dangerous for PCs to get to ranged enemies, whether that is because brutes and soldiers are blocking their paths or because the terrain is hindering them in some way. Perhaps there is difficult terrain that skirmishers and flying enemies can easily skirt around, but PCs are slowed down by.
The main goal of terrain in general is simply to entice the pieces on the board to move around, which is a big part of keeping an encounter dynamic. Sure, PCs might think it's a good idea to stand in front of a doorway to bottleneck enemies, but what if there is a holy altar in the middle of the room that grants creatures adjacent to it attack and/or damage bonuses? What if there are hostages in the room that need to be rescued? What if monsters in the room will alert their friends unless dealt with swiftly and their escape routes cut off? What if you simply sprinkle a liberal amount of enemies with AoEs in your monster repertoire (that'll teach PCs to instinctively scatter!)? Be creative, and take lessons from encounters that may not have gone quite as well as you expected!
There is one more subject which I would like to address in this post, and though it's mostly unrelated to terrain it's something that doesn't take much explanation (and thus doesn't warrant its own post). It's a good idea to plan at least some of your encounters to occur in waves. That is to say, the PCs encounter a group of enemies, and then in later rounds "reinforcements" arrive. This is a common tactic to use with minions, since minions provide a good excuse for there to be many enemies on the map. Waves are a great tool for several reasons:
- They allow you to fine-tune encounters on the fly. Are the PCs destroying your enemies far too easily? Increase the number of monsters in the second wave, or even if you didn't have another wave planned have one show up anyways! Are the monsters giving your PCs more of a beating than you expected? If you had another wave or 2 planned, consider reducing the number of reinforcements, or eliminating them altogether. The whole point is that you don't put all your cards on the table in round 1.
- They allow you to create larger-scale battles. Focus fire can really kill PCs, but if you expect a few monsters to be dead before a second wave shows up you can afford to plan more difficult encounters since the PCs won't actually be dealing with the entire encounter at once. Essentially you're refraining from "alpha-striking" the PCs in round 1. This is the same logic that makes the controller role tick, except controllers are the ones forcing the DM to act with fewer monsters.
- They can dramatically alter the tactical landscape, which can be great if the encounter has stagnated for whatever reason. For example, if the ranged PCs are sitting comfortably behind the frontline "tanks," simply have a second wave appear from the opposite direction! Now the party needs to form a second frontline, or somehow delay the advance of one force or the other.