Keep it real
Your game of choice might be fantasy, but it still has rules of nature just like anywhere else. Generally, the laws of physics apply (unless magic breaks them) and people still need to eat. So don't create worlds where lockjaw applies in 10 seconds and thirty foot goblins with horns are standard "bosses" (no kidding, one of my friends actually did this). Perhaps some of this might be of use in an extraordinarily magical area, but not as a standard first level adventure! This means doing some research. When you plan to have a disease strike a village, read about real diseases and perhaps adapt one of them to use. One of the most important parts of any game is making the players believe they are actually in another world, at least for a moment. Think about what the Gnolls eat in a dungeon-not just their hit points. Perhaps there is a large population of rats, caused by the larger-than-usual amount of carrion (which comes from the creatures the Gnolls killed, making this loop come full circle). You can then describe rats swarming the dungeon- perhaps even putting in some rat swarms as a random encounter. If you do this sort of thing often, your world will become much more believable- which is always what we're after, right?
