(Note: this is a re-post of something I originally published at the end of here.)
The DM is your buddy. He's there to facilitate your fun. The DM is not your enemy. He doesn't play to 'win' or to 'beat you down'. Or does he?
Claiming that there can be no adversarial relationship between the DM and the players is, if you ask me, nonsense. In any situation where there is a potential for failure, it is the DM's default responsibility to execute the challenge to the best of his ability. Most likely, this be a combat, but it could as easily be a skill challenge, a tricky bit of diplomatic roleplaying, or even a trap. Whatever the situation, he is doing his players a disservice if he doesn't use all of the options at his disposal... and use them well.
This means preparing the encounter, learning about the monster's abilities, and sparing a few minutes of thought for group tactics and the best use of the terrain. There isn't really any excuse for not doing that, unless the combat is improvised or has migrated into unexpected territory. 4E, even more than 3.5, requires this of the DM because monsters are so much more varied now, each one with multiple exceptional attacks, many of which synnergise both with itself and the abilities of its allies. Put together, all of this can change the nature -- and difficulty -- of a battle quite dramatically.
If the players are steam-rollering their way through encounter after encounter, they will get bored, even if some of them won't admit it. One thing, though, is guaranteed: the DM will get bored. Believe me I know. Bored, and frustrated. And more and more inclined towards unhelpful and maybe even uncharacteristically spiteful behaviour in pursuit of the simple task of getting to play an encounter to its full potential.
Nothing in D&D -- and I mean nothing -- frustrates me more as a DM than a confusion of circumstances in which monsters or villains barely get to put their abilities into action, where they become walking sacks of hit points crushed by the PC's. Sometimes this is down to luck, or simple surprise; sometimes tactics, intelligence, or team-work play an important part. These situations are okay because they're either out of your control, or the players are being rewarded for good planning. In fact they're more than okay... they're a pleasure to run and good fun to watch.
But when the monsters are being crushed simply because you didn't read the encounter, or because you didn't spot a hopelessly broken mix of monsters, or because the monsters themselves are simply not appropriate for the party... that can be disheartening. Good preparation is the only escape from this grim prison... and thankfully 4E makes it easier than ever to get that prep in.
The DM-player relationship, like any relationship, is a complicated one defined by context. The bottom line is that in D&D, rewards without challenges are dull for everyone involved. If they weren't, the game would consist of time-trials to see who could build a 30th level character fastest, instead of the complicated dance of mutual story-building and butt-kicking that the game is renowned for.
At least one of the DM's jobs is irrefutable: provide a challenge. In this battle of wits, it's 5 against 1, and he needs all the help he can get.