How can I be a great Dungeon Master in Dungeons & Dragons 4e?: originally appeared on Quora: The best answer to any question. Ask a question, get a great answer. Learn from experts and access insider knowledge. You can follow Quora on Twitter, Facebook, and Google+.
General DM advice
Dungeons & Dragons is a social contract, everyone comes to the table with certain expectations and when they don't meet is when you run into problems. Some people expect the DM to dominate the narrative, some people expect to have a lot of agency in where the plot goes, conflicts arise when everyone's expectations don't match up.
There's also setting and genre expectations as D&D can be anything from going down dungeons to stab orcs to negotiating with the high council on trade regulations and everything in between. This is not an issue of rules but of the social contract, so just work that out with everyone before the game starts so you don't wind up with a bardic diplomat player disappointed that all he gets to do is stab brainless skeletons, or a greataxe wielding barbarian player that really doesn't want the duke's ball to last yet another game session. Also periodically check up after sessions to see if players are getting what they wanted (or even pleasantly surprised with more fun than they expected).
Make sure your players are aware of composition of ranged and melee characters in the party. If you have a party of 4, with 3 bow users and one sword guy and they get attacked by a pair of owlbears, then that one sword guy risks getting double teamed by the owlbears if he wades into melee range by himself. If it was four swordsmen vs 2 owlbears though then it's the sword guys each double teaming the owlbears, drastic difference in expected tactics.
This advice carries over to most any kind of tabletop RPG.
4e specific advice
One of the goals of 4e was to make fights a little longer so the player characters can show off more of their cool moves... but a lot of the early monster designs have too many hitpoints and do too little damage. You run into situations where the party knows they've won the fight but it'll take another few rounds to actually bring the opposition's hitpoints to zero. The latest monster book, Monster Vault, does a good job cutting down monster hitpoints while increasing damage. A lot of my advice will be about keeping the combat portion of the game fun, dynamic, and fast so the players who prefer to be at the duke's ball can get to that part of the story faster too.
It's also pretty easy to reskin monsters in 4e as it's largely about changing the way they're described. So something written in the book as a spiked chain swinging hobgoblin can be revamped into a drow ninja with a twirling kusari-gama if you felt like it.
Same with player characters too. One of the characters I played as in 4e was a Bladesinger trained in stealth, so I said "he's a ninja".
Skill challenges, the way they work is you have to make X successes before Y failures, I dislike that as it discourages people with a low skill modifier to contribute as they increase the chance of failure. I recommend this simple fix: instead of 'x successes before y failures', make it 'X successes in Y turns' with failures not contributing to failure. This means that even someone with a low skill modifier can contribute with a lucky role.
As for how that works, here's a sample skill challenge of a burning house. The PC's have 3 turns to hit X successes until the house collapses. Every success means they're able to retrieve some valuable or lost kitty. So the athletic fighter says he's barging in and kicking down doors, the acrobatic rogue says he's flipping up a nearby storage shed and entering through the 2nd story window, and so on.
Though 4e has roles like 'defender' and the defender has perks like higher hitpoints, defences, and more healing surges, that doesn't mean the defender should be the only guy getting hit. If the defender is the only guy getting hit then the moment he runs out of healing surges is the moment the party has to go back to town and sleep at an inn.
Healing surges are there to make sure everyone can recover from a few scraps, this means that the wizard can take the occasional hit but refill for the next encounter. So for you as the DM, don't pull your punches, have your monsters fight as smart and tactical as they should and go after the softer targets if they can, this lets the defender characters use their defender mechanics actively. The alternative is orcs making a beeline into the paladin's shield while the sorcerer sets them on fire and that's pretty boring.
4e also focuses a lot on dynamic combat with lots of movement. If you're fighting on a flat featureless plane though you lose a lot of the purpose of 4e class abilities. Try to fill your scenes with interesting terrain for the players to interact with. Thunder wave as a wizard is cool, but thunder waving an orc into a blazing pit of coals feels way cooler.
4e also designed monsters in a way where they expect PC's to have +1, +2, +3 weapons and armor at certain levels. 4e also expects characters to have the expertise feats to enhance their accuracy. This means that a level 10 PC without such a weapon or feat will be wiffing a lot against level 10 monsters, that's no fun. I highly suggest you instead give your PC's a free expertise and free enhanced defense feat at levels 1, 10, 20 as a replacement for that.
So with all of that in mind, enjoy D&D4e!