Your settings will play a big role in getting nice picture. If you're shooting at 24 frames per second ([email protected] for example) you'll want your shutter speed to be roughly double 1/50. If you're shooting at 30fps you'll want to be at 1/60. if you're new to video remember that as a rule of thumb and don't deviate from it. There are reasons for using a slower or faster shutter speed but in day to day use it isn't going to be required. I'm not too familiar with the 600D but do some research online, there will probably be an exact way to set up the in camera sharpness, saturation etc. Basically you want the image in camera to have the most dynamic range and the least amount of sharpening (you an sharpen in post). I don't know what lens you're using but if it's a kit lens you can't expect fantastic quality out of it. The CPL may also be the culprit if it is a cheaper brand.
Hope that helps!