There are many schools of adventure design in RPGs out there but what intrigues me the most are campaigns where players scale their endeavors to their capabilities. For this to be possible the ref needs to create a campaign where the adventures are buikt in an environment where the bad guys want to win and don't simply exist as exp piles and challenge bumps.
Do you have to tell the players that the Citadel of Lord Black who has ruled Darkoria for 1000 years and is guarded by the undead remains of his 2000 sons might be a tough or overwhelming challenge for a group of 1st level characters? Is a tower with no more then 3 stories and a half dozen horses corralled outside likely to be appropriate challenge?
The players should be able to make the choice. The bad guys should fight back within their capabilities. If the PCs manage a single raid into Lord Black's citadel and come running out with a couple thousand in loot they deserve it but if they return three days later to do the same they should die or maybe Lord Dark just isn't all he is built up to be.
The clever traps and guardians to the treasure vault should be built to be accessed by Lord Black and his trusted minions, not an adventure and loot path for the PCs. If the PCs walk into this vault and discover a charmed green dragon guarding his loot and they weren't' expecting to do so, they should die if they don't run fast enough. Who would you leave guarding your loot, a green dragon, a minotaur or a band of kobolds?
If a player owned a castle and it were under attack would he make sure to have concentric circles of defense where his absolute weakest forces were on the furthest edge and be sure only to commit his stronger forces after all else was lost and would himself avoid the invaders until the last moment, or would he lead a reaction force to smash down the invaders daring to pilfer his hard won loot? The bad guys should react as a player would: with ruthless sociopathic-homocidal overkill and the players should expect that.
The bad guys want to win and a ref should build adventures that keep that in mind. Yes it sets the bar higher for beginning play but that's good. Of course one must be careful in presentation and make it very clear what power levels one is dealing with or beginners will throw themselves against threats they can't possibly overcome and possibly become discouraged. There's always the chance players will become better at the game and be able to beat the bad guys even when the deck is stacked against them by opponents who want to win.
Are the players returning to the same base time after time when raiding the Tunnels of Terror? Have the bad guys ambush them on the route to or from the tunnels, even have the bad guys strike at their base. It can be pretty embarrassing to return to the local Inn to rest and discover it's been burnt to the ground.
Are the players tasked with recovering the 6 segments of the Gilborax? Is the villain tryign to do the same thing? Have the villain use the PCs to make his life easier.
Are the players tryign to stop Lord Black from invading Heartland? Let them try, but he will not delay his plans unless the PCs are a real problem and if the PCs are a real problem their destruction becomes part of the invasion plans.
Just a little friendly advice to my fellow refs/DMs/GMs out there: build adventures with bad guys that want to win.