Sure. And how do you decide what it is? How do you decide how many goblins live in the lair? How do you decide what traps the tomb is full of?
If I'm using a canned module I'll just find something that a) suits what I have in mind for an adventure and b) is somewhere around their level (playing 1e gives me lots of leeway there) and let the module figure it out for me. If I'm putting the adventure together myself I put in whatever might make sense for the party's level (again, lots of leeway) and go from there.
How do you decide how the DC and number of successes necessary to have successful diplomacy with a dragon?
I don't. DC and SC are not in my game.
If they want to get all diplomatic on the dragon then they gotta get all diplomatic with me (in character), and we'll see where it goes.
Sure. But whether you're sneaking past the ogre, or killing it with swords, or recruiting it to your side, you're overcoming a challenge. These challenges might even have different XP values, since they may be of varying difficulty or complexity! And if you can account for the danger they pose to the party, you can build your own adventure without having to be an old hand at the thing already.
All true.
Wouldn't be nearly as much of an issue had the last few editions had a decent series of canned adventure modules to back them up at launch - we'd be able to advise new DMs to use canned modules for their first few adventures and keep our consciences clear in the process.
But realistically, a new DM probably should stick to canned adventures for a while, until she learns the ropes (says he, whose first experience at DMing was an adventure I'd designed myself - badly - which I then proceeded to DM - badly - why my players stuck around after that remains a mystery to this day).
All it means is making sure the party can't ruin an individual encounter with a certain effect,
Unless it happens all the time I don't mind the party occasionally pulling a rabbit out of their collective hat and nuking what would otherwise be a tough encounter. Sometimes you just do happen to have the right answer. But sometimes you don't...
and that each party member can add to each encounter in some way.
This I don't care so much about. Round-by-round or encounter-by-encounter or even adventure-by-adventure contribution balance is irrelevant, as long as it sort of evens out in the long run. And it usually does, with any long-term imbalance usually due to the players involved rather than the game mechanics.
Sometimes there's going to be situations in which you are useless*. Other times you're going to be the only one who can get it done. Fact of life, just like the real world.
* - I find this comes up more often the further away from the core 4 a class gets. The core 4 can usually find a way to do something useful just about all the time.
Lanefan