Comfort is of the utmost importance when selecting your dream campsite, but so is following the rules. The specific rules of each national park, national forest or state forest are different, but most agencies typically ask that visitors camp at least 200 feet from the nearest water source, both to protect sensitive plants near the shoreline and to prevent people from contaminating the water. That said, you'll likely want to set up camp close enough to a water source that you can easily fetch some for drinking.
The best place to set up camp in most areas will be generally flat, not too bumpy and with good drainage. Sleeping in a tent that is set up on a steep slope is never a good idea, because you'll likely slide downhill while sleeping. However, you also don't want the ground to be perfectly flat, because in case it rains you don't want the water to pool beneath you. In the event that it does rain, it is, of course, better to camp on higher ground.
Although lush meadows look inviting for backcountry camping, it's usually better to camp in somewhat thicker vegetation. Why? In the first place, it's better not to be the tallest thing around in case of a thunderstorm. It's also nicer for other people who are camping to see what appears to be a "wild" setting. Before setting up your tent, be sure to clear the area of any big rocks, branches or twigs to make the ground more comfortable to lie on.