Documentary filmmaking is specific in its execution and needs. It is not the same as narrative filmmaking because much of what is filmed is not planned explicitly and you often have to continue filming until you find something that you can use. What is captured is often more important than the quality of the image, and through this you need a camera that can maintain a standard image quality but is more appropriate to the needs of the filmmaking.
At this point it is irresponsible to shoot on anything except digital. Film is much too expensive to film a documentary with, especially since the image quality is not going to be quite as critical, filming continuously will be too expensive due to the price of film stock, and high definition has taken the place of audience expectation. Any choice other than digital video, or digital file storage, is a bad one that is archaic in nature and a waste of money.
Find a digital video camera that is going to give you options but remain inexpensive. If you can find one that will allow you to use either digital storage or mini DV tape so you can go back and forth depending on facilities. Try to find the best optical zoom possible because you are going to have to track distance images spontaneously. You may want to actually try and find a camera that has good automatic or preset features so that you can use them in quick situations, such as fast light and location switches. Over all you want to get the best image quality that you can, but also in a way that is going to be responsive to given situations. Sound support is important so make sure you have the ability to record on multiple channels with several different microphones. This is going to prove to be especially important during sit down interviews.
The type of documentary you shoot will also dictate the size you want. A large shoulder mounted camera will provide a more stable image when on the move, but a lightweight one is easier for travel. Think about the kind of videographer you are, whether or not you want every image or more clear ones, and choose a camera that is somewhere in the middle. Always remember that if it is too light it will be hard to manage, but an excessively heavy one can be useless in real world situations.
Brand does not matter as much as features, but you should look into companies that have a good reputation. Sony and Canon have proven themselves in the digital video revolution so they are good, but often over priced. The best way to choose this is to look at the camera itself and imagine using it in a few simulated situations.
You may or may not actually want an HD camera, mostly because it will be dramatically more expensive and you may not have the facilities to handle HD footage. If you know that you will already be able to do your entire post-production, including the entire editing workflow and Blu-ray/HD DVD authoring process, and then try it out. Otherwise you should just go with the best resolution camera you can get in the Standard Definition field.
Over all documentary is a diverse type of filmmaking shifting from field production to staged interviews to beauty shots to reenactments. You want a camera that is going to be satisfactory for all of these needs, even if you are going to have to bring extra ingenuity during post-production.