I think the same rule applies to just about any genre of entertainment. Most likely, only about 5 percent (or less) of any genre is going to be good quality. The rest will be terrible and only contribute to poor image stereotype problems.
i.e. comics, science fiction, action movies, etc.
That said, I agree with most in this thread, that the massive majority of anime is crap. Whether its a lost in translation thing or just general Japanese wierdness, there's a great deal that I just don't "get", and its turned me off to anime in general. It takes a lot to get me to watch anything nowadays. Only after frequent stellar reviews and good word of mouth will I even consider wasting my time.