When a DM sends the PCs on a specific mission, I think you have an obligation to play fair. If Ganmerlinster sends you out to retrieve the Great MacGuffin the expectation on the part of the players is that the beardy guy won't ask you to do something you can't handle. It might be a tough gig but no DMPC should ever send the party on an unpossible quest. (And any DM that sets up a quest where only a pet NPC can finish the job ought to be tied up in a burlap bag and beaten with sticks.) To me, this issue goes straight to the trust and responsibility implicit in the DM role.
But the Dungeon is a different matter. I'm talking about the classic big, sprawling, multi-level underground complex, places like Castles Greyhawk and Blackmoor. I feel a different sense of obligation when it comes to these nightmare underworlds. For one thing, I don't feel like the DM has to be fair in the sense that all challenges are meant to be overcome by the PCs. If first level PCs take the elevator down to level 5 whatever happens next is on the players' heads. No one should ever be in a big, multi-level dungeon and say "We can take the Troll King! The DM wouldn't throw him at us if we couldn't handle him!" Sometimes in a big dungeon enviroment the right thing to do is to run away to live to fight another day, preferably after leveling up and buying some scrolls.
If I'm reading him right my Prussian homie Settembrini looks at these too different states ("Challenge but don't overpower the PCs." and "If the PCs tug on Superman's cape it's their tough luck.") and sees too different styles of play. The first he might call Tactical Gaming, which emphasizes setting up fair fights and pushing around the pieces on the game board. 3rd edition D&D really brought Tactical Gaming to the fore. The second he calls Strategic Gaming, which hinges on taking unfair situations and working to overcome those handicaps through smarter outside-the-box thinking. Earlier editions of D&D (going back to OD&D, Basic/Expert, and 1st edition Advanced) maybe weren't always clear on the importance of strategic play, but they were written implicitly with that play style in mind.
Maybe Set is right and D&D has (at least) two distinctive styles of play, but I would suggest that a hybrid approach is possible and perhaps even preferable. In my experience few players want to constantly face the challenge of strategic operations. I know personally there are many nights when I just want to the DM to line up some orcs to fight. But to know that every encounter will have the CR crafted to your party's level eventually leads to boredom. I relish occasionally finding myself in over my head. One of gaming's great thrills is to escape an untenable situation by the skin of your teeth. Even greater is the satisfaction of going back prepared and giving the bastards what for. Hey, undead dickweed! We're back and we brought wooden stakes!
I started this blog entry as a way of putting the players of Beyond Vinland on notice. Don't get cocky. There may be balrogs at the bottom of those stairs.