Some people just buy the first dehumidifier they see that looks good. If you spend a little more time looking at what options are included, you can be sure that you are getting a more useful machine.
Here are some features to look out for:
Auto shut off when the water storage tray is full, or better still, a continuous drainage system that requires no maintenance at all. These usually require a floor level drain in the room that they will be used, but if that option is available to you it will save a lot of time and effort in the long run.
Some sort of visual indicator that the water tray is full. (Assuming that you don't have a continuous drainage system).
Built in humidistat (humidity meter / hygrometer) that can automatically control the humidity of the room. This is a vital feature to prevent the air from becoming too dry.
For larger units, wheels make them much easier to manoeuvre.
A frost sensor that will switch off the unit if it gets too cold.
An on/off switch. This may sound silly, but without one you will need to be able to reach the main socket, and that is not always convenient.
Extras such as an air filter can be useful to some people, particularly allergy sufferers.
Variable fan speeds. (most dehumidifiers have them).
Low noise levels. Everybody likes a bit of piece and quiet.