While I never hated Dawn at all, it wasn't until now that I actually find myself liking her. Currently doing a series rewatch with my mother and we just finished season five and I found myself defending Dawn to her quite a bit. I don't hate Dawn at all, I think she was a good edition to the cast.
Dawn's life is just kinda sucks. First you find out that you aren't real, all of your memories are fake. Then a little while later, your mom dies, then your sister dies. Kind of a crappy year, huh? So she's moody and a bit bratty, hey you know, I think it's understandable she flipped her lid a few times. I was moody for lesser reasons when I was younger.
That being said, I think the thing is there are a lot of times in s5 where Dawn is written as if she is younger than she is, but it's little things. Like the stuff she would say to Riley/Buffy's friends, the questions she would ask, etc. It really didn't seem like a 14 year old, and I'm pretty sure they intended for Dawn to be younger than that but chose MT, which I'm glad they did. I'm willing to look past these little things because to me, MT is great for the role and seems to mesh with the rest of the cast flawlessly, almost like she had really been there the entire time. Chemistry with SMG was fantastic and well as with everyone else; I really felt like she had a long relationship with everyone. MT just nailed it all of the time.
I love how the gang really treated her as if they were her little sister too. Made it even more of a family unit 🙂
Also, I loved her relationship with Willow and Tara too and wish we could have seen more of it. Add that to the list of problems for Dawn (the 2 people that were there for her after her mom and sister died...well Willow kinda goes nuts and Tara dies. Poor Dawn.)
Post s5, the other issues. Her not having many friends, she does, she references them. If my older sister and her friends were saving the world every other week when my friends were talking about highschool stuff, I'd probably hang around big sis too. In s7 she seems to have grown up alot and becomes a rather cool character and one of the few things I really enjoy about s7, if my memory serves (guess we'll find out when I hit it in the rewatch).
"Get out get out get out" aside, Dawn's not so bad to me.
I don't know why, but the scene where Dawn threatens Spike about hurting Buffy just popped up in my head, I had forgotten about that, and holy crap THAT is a great scene if my memory serves.