Not in fangirl-speech, because this is a question only sane people will ask themselves.
On Confronting Sexual BehaviourEdit
It has been said by more than one professional in the literary sense (Stephen King, to name one) that Twilight allows young girls to experience love and sex in not a very sexual manner. To put it bluntly, when Edward so much as touches Bella the description the latter gives of that feeling is like porn. So simple face touching becomes 'face porn' in Twilight.
Sex is never directly adressed, not even in Breaking Dawn when Bella becomes pregnant. It is however heavily implied that there is sex, and that there is a lot of it. But young girls don't have to experience things they are, according to Stephenie Meyer and her brother Seth Morgan, "not ready for", i.e. sexual education or "that which significantly decreases the amount of teenage pregnancies". (Isn't it therefore kind of ironic that Bella got married and had a child at 18 years of age?)
Research (Or Lack Thereof...)Edit
Not many people consider this, mainly because they (rightfully) think it is stupid that Meyer didn't bother to research the topics she wrote about, but the lack of research makes Twilight an extremely easy read.
If you've decided to tackle this book for all it's (not) worth, you might've noticed that despite the occasional annoyance, there has never been a thing you had to read twice due to not understanding. Sure, you might be baffled at some things Edward does to Bella (abuse), but it's not like you didn't understand said action.
The lack of research means that you don't have to remember anything to understand the book. You can literally start with the last book and skip the previous three, and nothing will ever be unclear to you. In fact, that would probably help, considering Meyer often contradicts herself.
The Lack of a Plot, As Well As Character DevelopmentEdit
Since the course of the plot (as well as side plots) often cause the characters within a story to develop, the same explanation applies.
Twilight's plot feels very lacking to experienced readers, since they are used to plots which cause a lot to happen within a novel. Twilight's main plot for the first three books happens to be Bella and Edward's "struggle" to be together.
Let's sum this up a bit:
The first book: Plot contained the "difficulties" of Edward being a vampire, which only existed in Edward's mind. Subplot contained Bella "nearly" getting eaten by James.
The second book: Plot contained Edward dumping Bella for no real reason whatsoever. Then when he thinks she is dead, he wants to commit suicide. Subplot is rather unclear since it seemingly connects so much to the main plot, but Bella triggers Edward hallucinations by "living on the edge".
The third book: Plot contained a half-arsed love triangle between Bella, Edward and Jacob. We were clear on who Bella was going to choose, because she was engaged to Edward. Subplot contained what should've been the main plot, i.e. the army of newborn vampires of Victoria and Riley. The subplot did NOT affect the overall saga in any way. It is promptly forgotten about.
The fourth book: Plot in this one is by far the worst. Bella and Edward have a baby, The Volturi wants to kill it because they think she is an immortal child that can spill their secrets. The werewolves also want to kill it when suddenly. . .Jacob falls in love with it! And so ends a half-arsed love triangle in peace, and Meyer has no more effort left to put into this saga whatsoever. Note the lack of a subplot.
As you may have noticed, is that the plot was very easy to summarize. It can actually be done a lot shorter, if one considers character development: Edward and Bella are the same by the end of the "saga" as they were when it first began, except Bella is now a Meyerpire and they have a hellspawn together.
For young girls, this causes no effort on their part, tracing the footsteps of the original author. Compare Twilight to something with a massive plot and tons of subplots, such as 'A Song of Ice and Fire', where you are left scratching your head after something happens. Twilight has no such moments.