^ This. This SOOOOOOOOOO much. Sorry Ran fans- it's true.
That's the thing- there are no negative things to focus on. She's a MARY-SUE. She has no "official" flaws. People hate her because Gosho made her too perfect. The only flaws people can find can be argued and messed with and formed in so many ways. She doesn't have Haibara's flaw of not trusting people, Heiji's hotheadedness, Ayumi's jealousy issues, Kaito's lack of modesty and fear of fish, Genta's food addiction, Mitsuhiko's insecurity, Kazuha's lack of logic (jumping to conclusions) and possessiveness, or her dad's alcohol problem (though it would be HILARIOUS to see that that was her hidden flaw all along- she hates her father drinking because then SHE wants a drink! XD lol). She's basically a fan-character that was made official.
Mary-sue's annoy people- especially when they're one of the protagonists.
It's not that they focus on the negative things- it's that there isn't anything negative THERE!
While it is most likely that if you support AiCon you hate Ran, that's not true.
I'm an AyuCon, and I had no problem with Ran but now, 600 episodes and 800 files later, when she STILL hasn't revealed ANY flaws, I strongly VERY VERY VERY STRONGLY dislike her and a huge HUGE portion of her fans.
If she had been revealed to have any of the flaws above then more than HALF of the people who hate her now would be GONE.