I've had the opinion for many years now that NPCs are the glue that drives all design.
Think about it.
What we refer to as mechanics are almost always character-centric. In other words, mechanics are there to build NPCs.
Now what about setting? What makes a setting memorable? Places can, but mostly it's the characters who interact with the PCs. In other words, NPCs. Are the PCs likely to remember the weapon shop or the crude, smelly weaponsmith who told them the not-for-mixed-company halfling's daughter joke? The bar they went into or the Thief they got into a knife-fight after their fourth drink?
Now let's look at adventures... By now I'm sure you know where I'm going with this.
While adventures are an area where plot is king, I still contend that the presence of a few interesting NPCs can make a standard dungeon crawl into something far more. Think about the moathouse in Hommlet, the starter adventure building up to the Temple of Elemental Evil.
Now what *I* remember about the moathouse is the "new master", Lareth the Beautiful. In fact, he appears in Monte Cook's sequel, Return to Temple of Elemental Evil. So an otherwise vanilla dungeon for low-level characters is suddenly memorable.
For that matter, try to imagine the Temple of Elemental Evil *without* Hommlet. Suddenly you've gone from playing the greatest module of all time to playing Diablo on the table-top.
Now I love Diablo but my point is, the presence of all those interesting NPCs in Hommlet, with their politics and their agendas, makes TOEE at least TWICE as good a module.
So what does this mean for Dungeon Masters?
Well, what it means is that designing interesting NPCs should be job one. When I create a campaign world, I like to have a map. I know you don't need one before the start of a game, but I feel more comfortable knowing the lay of the land in macro, meaning I at least want a continent worth of map.
Once I have that done, my #2 priority is NPCs. Some will be allies, some will be enemies but they will be the fuel for the engine. From there, plots just naturally seem to follow. If an NPC the characters have taken a liking to gets into trouble, that's not an adventure you need to sell them on or cajole them to undertake.
And what's more, it makes your campaign feel like a living, breathing place. Much more so than yet another quest for the Elemental Widget of DOOM!
In short, NPCs provide adventure material and are the best way to make your campaign seem like a real place. Try it. I think focusing on NPCs will improve your campaigns immensely.
And if you'd like a little help creating interesting NPCs, you might check out NPC Essentials. It's the best selling product in RPGObjects history. I didn't write it, but I heartily endorse it.