I can't help but wonder how many people who claim to hate Keller because she can't fight really hate her because she's paired with Mckay in canon and therefore gets in the way of McShep
I suspect that may be the reason for some, but not all. I think the biggest reason was because she replaced Carson, who everyone loved; including me. I didn’t hold that against her though, and I loved her and McKay together, so I don’t understand the Keller hate either.
She was annoying, plain and simple. But, in my opinion, it’s because she was played by Jewel Staite (am I really the only person who doesn’t like her?!?), whom I find obnoxious and particularly simpering in this role. Another actress might have made much more of it (and may have been able to make her a bit less whiny).
Also, I am not a shipper. Pretty much anywhere. Ever. So it’s really not that. At all.
And as for replacing Carson - as if anyone could. My feelings on that were with the writers writing him off. Someone else had to come in and I was open to that. But having someone who was unqualified and didn’t even have the confidence to try, being forced into taking that lead? When they could actually have got someone else from Earth? … well that’s the thinking that also gave us the characters in SGU… whom I also disliked very much.