Here's a whole line of reasoning which gets us to what we're facing. I have no idea if that's what Jack thought; it's merely an intelectual excercise.
Right. Now, the campaign started with a ghost and a paladin. The ghosts' strength is fighting anything corpereal, with a minor weakness against things with high Will saves and grapple modifiers. My character is a paladin - Best against evil creatures.
Now, a GM can always make evil NPC's, but it's easier just to use pre-made monsters. Since paladins are champions of spiritual good, the naturally opposed monsters are undead and demons.
Then we picked up a rogue - Ashtons' character. Undead are generally immune to critical hits, so even a low-level undead would still leave 1/3 of the party pretty much innefective. (Which would change if Ashton picks up an anti-evil weapon - Natural attacks can be enchanted). However, the current situation leaves demons as a good moderate challenge to the group.
Then, enter one mage - And one cleric. See where I'm going?
Of course, we probably will face other threats - But this is merely an intellectual example of how to calculate a 'Standard threat type'. It's also not the only possible style of GM'ing - Some GM's perfer a mix of encounters, some of which the party is unprepared for, by nature of class.
The purist Gamist GM, to explain, simply puts threats and plots down, and leaves it completly up to the players to avoid TPK. If the party accidently walks into the dragons' lair while the dragon is there, it's entirely up to them to avoid being toasted.
The Storytelling GM is the other extreme. If the party accidently runs into a dragons' lair at first level, it's because it's plot-related, and they won't get toasted unless they do something like try to steal treasure. And if it's actually not plot related, but an accident of where the party choose to go, it'll be plot related rather quickly.
There's more to it than that; for one thing, most GM's (Me, for example), seem to fall somewhere in the middle. And those aren't the only style of GM'ing.