Too short? Bald? I see nothing wrong with the fact that Aang is too short. I don't even see why looks matter. And Katara's not the type of person to judge, either. Besides, he IS going to grow up. And when he does, huh. Hopefully they'll have an epilogue in the future ^^
Too young? There's a two year difference between Zuko and Katara as well. And this is a different world. Obviously age doesn't matter if it's normal for a sixteen year old to marry.
And when SOME (Note the some) use the opposites attract thing with Zuko being a firebender and Katara water? First off, I think it's a poor reason to state why they should be together. Besides, isn't Aang a firebender, too?
There's also the excuse that Zuko lost his mother and his father hates him, so he needs someone like Katara to comfort him. Aang has lost a lot more than Zuko. Don't get me wrong, I feel bad for Zuko and all, but Aang lost his entire family. His origin. He then finds out he's the avatar and has to defeat the firelord or else it's his fault and the world ends. He can't be a normal kid. Zuko only his mother and we don't even know if she's dead or not. He wants to defeat his father, so he doesn't care about him. And I don't know about you, but I'd rather lose one family member than my whole family o_0
And I don't think Katara needs someone who's always unhappy and miserable. She needs someone who would make her happy and her life fun. I mean, if it weren't for Aang, her home would probably be destroyed and she'd be dead.
Aang as a little brother? Ugh, I hate this. Katara's motherly to everyone. It's not just Aang. Everyone. And big sister's don't kiss, hug, or give romantic looks repeatedly to little brothers.
I guess I just have my own opinions, but what I find more romantic is falling in love with your best friend. It's like if you knew this person for so long and then it suddenly hits you that you have feelings for that person, but you're too scared to admit it. And then when that person has the same feelings as you do. I don't know. I just find it more romantic than hating someone at first, and then falling in love with them. It's too quick.
So...yeah. That's why I like Kataang