1. She's the main character and the leader of the group. I've always been a huge fan main character because that's what the story's about. Well, also because generally I get the manga before I see the anime, and usually manga volumes begin with main characters first so I almost always develop a loyalty to them early on.
2. Mitsuishi Kotono. I love her! Almost as much as I love Hayashibara Megumi and Amuro Namie (who's not a seiyuu, but that doesn't matter)! I adore seiyuu.
3. She's got all those cool transformations and attack sequences, I love the Moon Crisis Make-Up sequence.
4. She's cute, actually, more like adorable. I like her hair, I wish mine were enough to put my hair into odango, but I'd probably look like a little Japanese rabbit on crack.
5. She's got a strong-will. I know that seems stupid to say considering she was such a baby in the earlier series, but she really does grow up near the end. Her strength is admirable.
6. She has Chiba Mamoru--he's sooooo cute! I'd love to have a boyfriend just like him, except not as cold. Mamo-chan strikes me as a cold kinda guy, but this is about Usagi, not Mamo-chan.
7. She's got history, I love that in a character, which is probably why I like main characters in anime the best. You get to hear about their past, their present, and sometimes their future. Everything basically revolves around Usagi-chan's life, and I think that gives you good insight on why she is the way she is.
8. She's nice! She never wants to just kill everyone, she wants to help everyone. She believes that everyone is good at heart... It's naive and stupid at times, but you can't be perfect.
9. She's fun: she's the most childish of all the Senshi, except maybe Minako. You're only a kid once.
10. Okay, I ran out of reasons.... Umm, let me once again state that she's the main character, and I love main characters, no matter their personality type. Yes, I do realize Usagi's a ditz, but she's got so many other redeeming qualities.