Read up on exposure and how light meters work. Light meters, as sophisticated as they are these days, are still dumb machines. Getting the right exposure is vital for good looking photographs, as well as understanding the limitations of the gear, such as dynamic range, and how to work around them.
Always use a lens hood.
Learn to use the in-camera image processing software or post processing software: color correction, contrast, and sharpening.
For hand held shooting the anecdote that goes around is to use a minimum shutter speed of 1/focal length of the lens, but it can vary depending on the person and the gear. If you can get the shutter speed up to two or three times 1/focal length you are much safer from camera shake.
Or use a tripod (sturdy enough for the gear), and cable release (or self timer).
Just like polarizing sunglasses, a correctly used polarizing filter will reduce glare, and increase saturation. There are lighting situations, like when a bright light source is in front of the camera, when filters (such as a protective UV filter) can flare causing a reduction in contrast and saturation.
Practice manually focusing occasionally, and don't be afraid to turn off the AF. Focusing is a skill like any other. Even if your AF works great, there are times when you'll do better to manually focus.