Honestly, it's beginning to feel like people hate Twilight, just for the sake of hating Twilight. The "monkey see monkey do" thing: everyone else is doing it, so they need to too.

I don't hate it, I just wish that the vampires didn't sparkle and the werewolves didn't look like puppies. Technically, they're lycans, too.

A good mix

For the most part. Lately, a lot of people are completely against whatever's not the object of their affection. Compare anything against their favorie book/movie/musician, and you'll get a large number of people saying how the other thing sucks.

There's also the popularity. All the dislike for Twilight is similar to the common dislike of pop music. It's popular and "mainstream", thus created for the inferior.

For me its popularity, fan base etc. doesn't matter. I just strongly dislike the whole story, this "vampire falls in love with girl" thing. And I also dislike almost every character on Twilight.
Why? I can't put my finger on it. Just the behavior, the way they speak, act...to me it's kinda boring and the plot is as deep as a paddling pool. And it's not that I hate vampire stories, in fact I really like them! I'm a big fan of The Vampire Diaries & Buffy but Twilight? Though I have to admit, earlier I was Twilight fan...but only of the books. As soon as the first movie was released, I kinda change my mind immediately, it opened my eyes. The movie was awful to me and suddenly the books were too. To come to an end...I just strongly dislike the whole "package", simply everything. Except Alice Cullen. She's adorable ;)

Well, with me it's because Twilight is:
-overrated (I mean, c'mon! I've seen millions of books and movies that are better and deserve more recognition that Twilight)
-sexist (Bella is a wimpy, whiny pushover who cannot even walk a mile without Edward)
-lacking of a solid plot (There is a very little amount of a conflict)
-cliche (This is probably the 1000th or so "Vampire Falls In Love With Human" storyline)
-unrealistic (Pretty much all of the girls I know do not act anything like Bella, and I know a lot of girls. Also, love doesn't work in real life like it does in Twilight.)
-lame (The "sparkling vampires" concept is one of the most retarded things in fiction)
-containing one of the worst fanbases ever! (A lot of girls have been turned into giggling, airheaded girls who like to masterbate about cold-skinned, abusive sparklepires and wolf men or rabid fanatic animals who try to drown people who don't like the series in toilets)

Every person is different.

I disliked it as a book.

actually, if not for the crazy fan i wouldn't hate it that much :/
it's just that the way they response to any argument are rude. and hell, i'm not a native English speaker but they can make my eyes burn.

The characters are like perfect inhuman robots (ex: Bella Swan). I like Alice, Jasper, and Carlisle though.
The plot is horrible compared to other stories (Harry Potter). Twilight talks about "sparkling vampires" and a "perfect" annoying girl who falls in love with both her best friend werewolf Jacob, and her sparkling vampire boyfriend Edward.
Harry Potter is about a boy who's parents were killed by one of the darkest wizards of all time and about how important bravery, friendship, and sacrifice is.
The main character Bella is also a HORRIBLE example for young teenagers. She's whiny, complains a lot, and can't live without her vampire boyfriend and werewolf best friend.
The main character Harry is a PERFECT example for young teenagers. He may have anger issues, but he's brave, and he loves his friends dearly. He puts others before himself, and he hates having people sacrifice themselves for him. Harry is completely selfless.

There are many more reasons, but I don't wanna state them 'cause then that would take too long.

the whole writing is a complete turnoff

I don't hate it though. Hate is a strong word.