This is something I could've put in a blog but I put it up as more of a disscussion if only because I want it to be a "Discuss the common reasons why people dislike Miku" thread. Anyway this thread derives from the fact that we get a fair number of people here asking this question despite it being explained quite a fair number of times. So Serza is here today to go through the number of reasons why people may hate her (besides obvious and simple reasons like trolling).
FYI : I am not saying that you have to dislike Miku with this thread. The intention is not to change people's opinions on Miku but to give a better understanding as to why people dislike her even if you do.
1) Producers use her too much/She has too many songs. This is the most common reasoning you'll find to this. Sure enough Miku's fame is what brought VOCALOID into the light, as well as providing companies inspiration to make their own VOCALOIDs. Despite this there is no denying that Miku even to this day has an overwhelming amount of original songs to her name that is far to superior to not only overseas vocals or unpopular Japanese vocals, but even when compared to some of the other more popular VOCALOIDs. But it's especially because she overshadows the lesser known/popular VOCALOIDs that people get frustrated with her. The majority of the time fans do understand that this isn't directly Miku's fault, some of them might even still like her voice anyway, but they'll still be annoyed with the overwhelming amount of Miku songs being made while they wait for their favourite VOCALOID to actually receive an original.
2) She's too popular/Fans give her too much attention. Being the reason why 1) happens, this is a fairly common reason too. As mentioned she is the one who brought VOCALOID to fame and naturally her fame rose as well. The issue people have with this is not the fact that it happened, but the fact that even to this day it has not died down, which in turn corresponds with this idea that she overshadows over VOCALOIDs (Which I keep mentioning but will talk about more in depth later). There is always going to be one that's more popular than the other, there is no denying or changing that fact. But people are annoyed with the fact that Miku has always been on top and there's not even a slight shift in her popularity. Again it's not so much the fact she's there but the lack of variety when it comes to what's popular bothers people; even if there is a newly hyped and popular VOCALOID they are usually set aside after a while for it to return to Miku.
3) She's oversahdowed. I've mentioned this a few times already but let me explain now why overshadowing is an issue. I'm going to take this in it's own points :
Is this all Miku's fault? No it's not, but in most cases this is the impact she has brought upon the companies attitudes towards VOCALOIDs, even if she herself didn't directly do it.
To sum up here :
Additional reasons :
4) Her voice is too cute/annoying/unappealing. There are many people who are put off by Miku's voice, simply because they personally aren't attracted to her voice type. Similary there are people who don't introduce potential fans with Miku because she can be a turn off to VOCALOID as a whole. On top of this high pitched voices like hers can even physically hurt people's ears (not because they think she sounds that awful but because the ringing in their ears can cause headaches, i'm no professional on such conditions but there's a fair bunch of people here who can tell you it happens to them).
5) Her design is too cute/moe. Beleive it or not there are VOCALOID fans who aren't fans of anime. At it's core VOCALOID is a voice synthesier for creating songs and people can be drawn in just for that sole reason. Naturally then they can find a design unappealing because it incorporates many common anime design tropes (Despite how original you think her design is she was designed with having anime elements in mind). Of course they can ignore the design since it's "the voice that matters" but no matter how you look at it the character is part of the product so you can't just detatch the character from the voice just like that.