I don't hate Ran as much as many people do, but that doesn't mean that I like her.
Reasons for that:
1. Like Parkur said above, she cries too much. About this reason, I personally think that it was because of Gosho's "failure" in drawing the readers' sympathy towards the character. In the past, from the beginning to the end of Vermouth arc, I actually felt sorry for her. I felt her pain and her sadness. I even admired her strength when she tried to hide her tears from everyone, and still acted naturally (Volume 33). But now, Gosho is just making her cries more and more annoying. She cries, but the way Gosho portraits it makes me hardly feel anything. I don't even see her sadness.
2. Gosho makes her role much less significant then it was supposed to be. She's the 2nd main character after Shinichi, but she has absolutely no role in the development of the main plot itself. The scene where Gosho made her jumped in and saved Ai was rather random IMO (I don't know about the rest of you but that scene made me admire Vermouth much more than it was to Ran).
Even for the normal weekly cases (which some people called "fillers"), her role is nothing more than to call the police when a murder occurs. Because of that, it doesn't surprise me when some people think that her image is more "faded" than other characters like Ai, Kazuha, Sonoko... or even Akemi Miyano.
3. One of the most important reasons is her personality. How can I say ? It's too "general". If someone asks me to describe the most unique (but positive) element I find in Ran and not in other char., I'll be speechless, since I don't even know the answer. That's right. You can hardly find things that are unique about her character, even when she's the 2nd main. (The fact that she cries too much or fears of the supernatural doesn't count because they're negative). I don't care about the recurring ones, but as the lead female character, having no (or few) specialty is also Gosho's "failure" in portraying her image.
Of course her highlight is always the "Angel" part, where she saved people who was about to kill her and such. If that was real life, then I would worship her like a goddess for such action, because that's something you don't see everyday. But this is IN Conan Universe, where every good character is likely to do the same thing. The proof is that if Ran hadn't saved Vermouth, Shinichi would have done it anyway (after all, the "there's no reason to save" speech was his). Also, if it hadn't been Ran following Vermouth and saved Ai, but instead another good character, they would also have rushed out to protect her from Vermouth... Therefore, Ran lost her uniqueness.
4. Sometimes, Ran used (or was about to use) her Karate without thinking, which only adds more negative points to her character. E.g. in Volume 55 (Eri's secret case), where she almost kicked her mother's guest right on the face even though Conan told her it was a misunderstanding. And in Volume 61, she "successfully" kicked at Okiya's face without even letting him finishes his explanation. Who in the world attacks people when she herself was the one who asked "Who are you?" just like 1 second ago ? (File 638 - page 8) She's lucky that he isn't Yusaku or Yukiko's acquaintance who borrowed their house.
5. Last but not least: Her hair-style.