How big is the area that you want to humidify? Match the stated output of the humidifier to the area you want covered. Output is measured in gallons; for example, a 3-gallon humidifier can put out 3 gallons of moisture per day. The specs for the humidifier should also say how big an area it can cover.
How long is the time period you want to run it? Although a 3-gallon humidifier will humidify a large area, you'll have to refill it frequently if it has only a 1-gallon tank. Also, if you want to run it overnight, you want there to be enough water to do so.
How maintenance savvy are you? Indicator lights alert the user when water is low or the filter requires maintenance. Even better, an automatic shut-off prevents the humidifier from running when the reservoir is dry.
What kind of water does your municipality have? If you have hard water, a demineralization cartridge that minimizes dust caused by the minerals left behind by tap water is a must. Some models come with a demineralization cartridge; in other cases, it's an optional add-on. Remember to replace the cartridge as scheduled.