Knowing the difference between bad lighting and good lighting can save you from wasting valuable space on your flash cards and tons of editing time. While the difference is subjective based on intent, good light is generally directional and casts some shadow that give the subject a sense of shape. Bad light either hits the subject flatly, creates unwanted shadows or misses the subject entirely.
The quality of the light is more important than the quantity. Whereas poor quality light will be bad no matter what camera you’re using, most current digital SLRs can take a great photo in low light as long as it’s good quality. Knowing what you’re looking at will tell you when to shoot and when to hold off (when in doubt shoot).
The book Light: Science and Magic: An Introduction to Photographic Lighting is a great reference for photographers of all levels.