Fresh from novelist boot camp, I can say that "It was a dark and stormy night" is considered such a howler because of the "was" followed by the double adjectives, both of which are poorly chosen. And then on another level, the whole line is a big cliche'.

It's easy to tell a good opening line from a bad one, as a reader. As a writer... ouch! The first line will probably be the very last thing I commit to in the final draft. So much is riding on it, and it's so easy to screw it up.