There be adventure...

I like the idea of the DM creating his own D&D campaign world. Putting his stamp on it. Knowing it inside out. Expanding it as needed.

On the other hand, there are plenty of quality published settings from which a DM can draw inspiration from or use as a baseline for creating his own setting. Perhaps just the maps are adopted from these published settings and the DM fills out the details. Maybe someday I'll feed the need to create a campaign world from scratch, but for now I am quite content with capitalizing on the wealth of existing campaign world material and customizing it where I feel the need and inspiration.

AD&D2E saw the publishing of many interesting worlds (Dark Sun, Dragonlance, Planescape, etc.) and maybe someday I'll use them for more than just inspiration. For now and for a Classic D&D campaign, the most obvious choices for serving as campaign baseline are Blackmoor, Greyhawk, The Known World (a.k.a Mystara), and the Wilderlands of High Fantasy. To summarize:

Blackmoor
I don't own much Blackmoor material, so am unable to say much about it. What I do have (First Fantasy Campaign from Judges Guild, and a pdf of the Blackmoor OD&D supplement) is more house-rule compilation than campaign world material, so will mostly serve as a source of inspiration rather than as a campaign world baseline.

Greyhawk
The Greyhawk world, as originally presented, is a container for the classic AD&D1e modules by Gygax and others (G1-3, D1-3, etc.). Though Greyhawk would later mature into a full-fledged campaign setting, it's my understanding that the early Greyhawk flavor is best experienced by playing those classic modules. If those modules are played (and surely some of them will be played if the campaign continues long enough), the campaign can't help but get a dash of the Greyhawk flavor. The World of Greyhawk setting material (1983) may also be mined to reinforce the Gygaxian stylings of the classic modules. Its colorful poster maps are inspiring just by themselves.

The Known World
The default setting for Classic D&D was first introduced in the module X1 The Isle of Dread, released in conjunction with the "Expert" rules set (1981). This early presentation is rather minimalist: A two-page b&w hex map of "The Continent" and a paragraph description of each region or nation. Not very impressive by later standards, but it served its purpose well, giving the DM of a Basic+Expert D&D campaign a springboard from which to build his own world. Perhaps the only distinguishing feature of the Known World was that each nation had a unique flavor analogic to some real world culture or era. Something for everyone.

In the late 1980s, TSR published the Gazetteer series of supplements detailing each nation of the Known World. Taken individually, these are fine supplements for adding depth to a campaign, plus they came with nice color poster maps. Taken altogether, though, they are somewhat overwhelming and at odds with the spirit of Classic D&D. My preference is to pick and choose which Gazetteers to include in the campaign. A favorite of mine is the Karameikos Gazetteer, a generic fantasy setting and home to many of the B-series modules.

Wilderlands of High Fantasy
I tend to think of Judges Guild's original (1970s) Wilderlands as less of a campaign world and more of a campaign tool. Maps and tables mostly. There are many great modules and city supplements set in the Wilderlands, but they seem to serve as examples of the diversity of a Wilderlands campaign, rather than as official components of the setting. I find the Wilderlands maps to be highly inspirational, in a sepia-toned kind of way.

Where does this all lead as I contemplate the world of my current D&D campaign? Really, it doesn't make much difference which setting serves as baseline, since I will be mixing and matching anyways. The large Wilderlands poster maps (all 18 of them) lend themselves well to being the "big picture" world maps. The Greyhawk and Karameikos maps and modules can fit into the generic structure of the Wilderlands with ease. Most importantly, the actions of the players will be the deciding factor in determining the growth and detail of the campaign.