To start I would like to say that I am not posting this to be mean or start any fights. I'm posting this because a lot of people ask me why I don't like her or simply assume it's only because I like Bellatrix, and I want to clear this up. Plus I'd rather just post a link to this article rather than explain it every time someone asks.
Anyways, the first and biggest problem I have with Molly is (yes the obvious one lol, I'll get it out of the way first) that she killed Bellatrix. I do respect her motives, her kid's life was on the line. But I still don't like it. I also don't like how she called Bellatrix a bitch before killing her either. And it really gets me that Molly acts like Bellatrix was completely heartless-Bellatrix would do just as much for Voldemort (and she has) as Molly would, but that's another matter.
Moving on to the less obvious; she nags her kids way too much. Prime exsample being her howling letter. That was pretty harsh. I'm no Ron fan, but I felt so bad for him, just imagine having everyone in the great hall laugh at you as your mom screams at you. Getting yelled at in front of the class is bad enough, but having your mother yell at you in front of the whole school? On top of that there was no 'are you okay?' No it was just a 'you could've gotten yourself killed.' Yes there is obviously a concerned mom behind that, but her tone screamed (pun intended) 'why are you behaving so stupid Ronald Weasley?' more than it did care.
Another example is the twins. The woman is nagging them all the time. I'm pretty sure most remember the "Just because you can use magic now does not mean you have to whip your wands out for everything" quote. All they did was teleport next to her and she's yelling at them.
On that note she was also very strongly against Wizard Wheezes, one of the story's coolest shops. If Fred and George wanted to start their own business I don't see why she'd want to stop them, it's a source of income.
One of my other biggest reasons is her treatment of Fleur. Molly was very quick to judge her; labeling her as a snob, a brat, and a prick. Molly had also bluntly questioned her love for Bill, claiming that she'd leave him because he became a werewolf. Or at least I think that was her. She also went as far trying to talk Bill out of the marriage and was very disproving of the relationship.
Then there's the fact that she seems to care more for Harry than her own kids at some points. Going back to the howler; she didn't even blame Harry or yell at him at all despite his involvement. And in Deathly Hallows (forgot the chapter name, think it was 7 Potters) she seemed to care more about protecting Harry than her own kids (no wonder George lost an ear). I get it, Harry is important, but her motherly instants should have said otherwise. I could imagine that most parents hold their kids as a higher priority than anything else. Narcissa put Draco before practically anything; she lied to Voldemort for him and made an unbreakable vow with Snape (mildly annoying Bellatrix in the process) for him.
That's about it for now, if I think of more I'll add them. Again, sorry if I offended any Molly fans, I'm just too lazy to keep typing the same response over and over again every time someone asks me why I don't like Molly.