When shooting people, making sure that you're shooting your video at eye level can be a great way to improve the look of your video.
For starters, your subject won't appear to be any shorter or taller than they really are. There are many times where you may feel shooting from below or above will add to the artistic value of the video, but for normal shoots consider this concept.
When we're trying to communicate with people, we like to look them in the eyes. And because we're trying to shoot our video as the eye sees, then it makes sense to shoot at eye level.
Perhaps you want to achieve a majestic effect where your subject towers over the viewers. This is a great example of how something as simple as the angle at which your shooting has a very emotional affect on the viewer. Think about times where you feel like you're looking up at whoever is on your tv or video screen. Often they're a person of importance or a hero (I see it a lot with sports video).
On the other hand, you may want to convey a feeling of smallness by aiming the camera down onto the subject.
You're main concern (if you're not looking for an artistic effect) is to make sure you're not accidentally making somebody look smaller or bigger than they really are. This will help people watching your video make a real connection with whatever you're trying to communicate.