Important areas:
Light: As wheatgrinder points out, lighting design is unbelievably important. Photography is about light first, and composition second. Even the most amazing HD camera in existence will deliver substandard-looking footage if your lighting sucks.
Motion blur: Your shutter speed should (generally) be set to twice the speed of your frame rate. Shoot in 24p with a shutter speed of 1/50th of a second (as most DSLRs can't do exactly 1/48th of a second).
Exposure and Grain: Since your shutter speed is fixed, you can only control your exposure via aperture (size of the opening in the lens the light comes through) and ISO (the sensitivity-to-light/sensor-signal-amplification setting). Small aperture numbers (big opening) let in the most light, but gives you a narrower depth-of-field. A high-ISO number (high-amplification) lets you shoot in lower-light conditions, but also adds grain. Use the absolutely lowest ISO number you can get away with.
Native ISO: Canon cameras have "native" ISO settings of doublings of 100: 100, 200, 400, 800, etc. All of the in-between ISO settings are either the lower ISO number amplified (increases noise) or a higher ISO diminished (reduces noise, but highlights clip 1/3rd of a stop sooner). This is why you'll find less grain/noise at ISO 160 compared to ISO 100 -- it's actually the native ISO 200 setting de-amplified a bit. The rule is: one ISO click above a native ISO amplifies (more noise), and one ISO click below a native ISO de-amplifies (less noise, clippier highlights).
Depth of Field: If your aperture is wide open (tiny aperture number) on a low-light lens, your depth-of-field might be very, very narrow. This can result in the wrong parts of your image being out of focus. Example: I shot a short film in candlelight, and a few shots were done with an 85mm lens at f1.2. My depth of field on my main character was roughly half an inch. I had a hell of a time just keeping his eyes in focus.
Bitrate: The number of bits per second used to encode the video into the h.264 format. h.264 is
extremely lossy (eg. throws away visual information), so always shoot at the highest bitrate you can. The default setting can get you blocky compression artifacts on smooth gradients, and most Canon DSLRs don't let you directly alter your bitrate. If this is the case with your camera, use the Magic Lantern firmware and the fastest memory cards you can afford -- it may take some trial and error to find the highest bitrate you can use that won't cause glitches in your footage.
Noise Reduction: Get
NeatVideo. Seriously. I'm not kidding. Do calibration shots and save those settings. You can get very clear footage out of fairly grainy sources -- super important if you're forced to shoot in lighting conditions that require a high ISO setting.
Color Profile: You should be shooting in a flat color profile like Neutral, Faithful, Flatt, or CineStyle (those last two are third-party add-ons). This will give you the most latitude (detail-retaining areas from darkest to lightest) in post for color correction and grading. Also remember to set your white balance!