I have been playing since the days of 1ed, and have been dming since then as well... and in that time, I have "created" numerous homebrew campaigns... and yet, none of them (except for the one I am working dilligently on now) needed the entire world created.
I usually decide what style of game I want (as has been mentioned before). Some of the genres were:
undead heavy (not Raveloft, per se... just something to make clerics and paladins drool)
pre-Greyhawk
mostly water areas
etc.
Then, I decide what monsters fit most into what I see in my mind.
Then I create one small town, and start the adventures there (and rarely in a tavern). I usually figure out a few adventures that I would like to run, and figure out how to get the party there, and by doing so, come up with recurring characters, towns, major NPC's and such. If any characters die, they are given a name change, and made into major NPC's. By the time we get tired of playing it, or the entire party dies, the world has a rich history and is a memorable place that I didn't need to kill myself to design.
Oh, yeah... one last tip...
Let the players take as much of the burden off your shoulders as possible, even when they don't know they are doing so. If your players build their characters with a backstory, then you can use that in building adventures, history, artifacts, etc... and those items will, in turn, lead to other bits of world building, which will- in turn- lead to... get the idea?
Players also have the bad habit of talking out of game, or saying things like "watch this be..." or, "I bet we run into (come across, land in, find)..." or, "If <this> happens, we're hosed". Use those pieces of ad lib from the players to help build your world.