at first i didn't like dawn at all, and i was known to dawn-bash. but as i've watched the series more and more, she become more tolerant to the extent that you can like her. true, you go back to season 5 and might feel she's still ridiculously annoying (which i still do), but she does grow, and by season 7 she's definitely matured, to the point where she doesn't need to be sheltered from things that go bump in the nite, and she can even partially take care of herself.
i think that most people come out hating her in season 5 because she IS so different from all the other female roles. but viewers have to remember, she's a teenager (in which case, they all act out and behave like that quite often), sister's do have their squabbles & fights, and plus she may feel like she's somewhat inadequate compared to buffy. buffy is given all this responsibilty and respect, and treated like an adult, and like any teen, dawn feels she isn't given the same. and of course she isn't an adult and hasn't been through what buffy has, but it's still the mindset of every teenager. also, people find it hard to like, in my opinion, because when buffy was dawn's age, she was pretty much already facing off with vamps and making hard adult decisions, but that's because her calling forced her to. sometimes i just think people just ease of dawn a little. she's kind of allowed that sort of behavior for that age...