High-def can give you one of two things. One - an amazing breathtaking image or two - a really poppy picture. HDTV's can be harder to get a good picture from a normal TV if you deviate from the factory presets. many people will turn up the brightness and sharpness to give them a picture that really 'pops' but in turn killing their picture. Most image problems can be diagnosed back to a bad source like basic cable or the TV's setting are just plain off. Popular Mechanics has the best layout of common HDTV image issues we have ever seen. They took an image of Jack Bauer from 24, picked at all the problematic spots and tell you how to fix 'em.
Even if you are the best calibrator in town, you might want to check out this PM article.