I have learned that expressing a negative opinion about a popular author means I'm jealous, and expressing a positive one is jumping on the bandwagon.
I've also learned I don't care.
Twilight hasn't so much taught me anything, but confirmed ideas for me. That lots of women really do like that romantic trope of obsessive love, that him being rich and cold, and she being poor and practical has been a magic combination for forever (Pride and Prejudice anyone?), and that character really does triumph over plot. I don't think Meyer did any of these particularly well. I think that she succeeded through a lot of timing and luck and coasted on the strength of the above scenario, but that is not the debate here. Amazing really the power of the good old fashioned love story.
(fwiw, I am having a P & P marathon on Friday! - the 1995 mini-series, not the Knightly film, which is quite good, but will never be as fab as the former)