The same can be said of not-so-great books. Reading poorly-written sentences or intros, observing awkward dialogue and thin characterization -- these are all great teachers. We can learn what not to do, in our own prose. "I will never write/say/do that!" <--we should file that away in our writers' minds.
Along those lines, here's a fantastic blog entry that reveals 18 "Don'ts" that I fully agree with.
Ultimately, I think it's about awareness - once we're aware of the pitfalls and weaknesses, we can do our best to avoid them!