Her sense of duty. Some people find her bouts of selfishness make her self-righteous when she talks about responsibility... but to me, it's the fact that she genuinely wants a more "normal" life that makes her truly heroic. And that she's not playing at "Hero" because it makes her feel important or good about herself... but rather, that she recognizes what she can do for others, what happens if she doesn't, and can't not be involved.
Her turning down Xander in S1 is about as nice a rejection as anyone will ever get, but her general unwillingess to avoid unpleasant confrontations is something I don't particularly like. In a number of her relationships, she will default into passive states - and a general lack of desire to directly state things if she can put them off. Sometimes it's a function of ambivalence, of not wanting to hurt feelings, or to find out she'd be rejected if people 'really knew what she was thinking/feeling' I'm not quite sure.
I'm less of a romantic about it than a lot of others, but I do like Angel with Buffy - noting that his curse and immortality make this mostly moot. The key thing is... I think he has a far greater understanding of strain her conflict between aspiration and responsibility places upon her than most anybody else does. It's a huge deal for her.
You can ship Buffy with a large number of characters from a number of shows - it just depends if you want to go silly or serious. In silly worlds, I can easily picture her having a brief, yet embarrassing dalliances with John Dorian (Scrubs) or Ted Moseby (How I Met Your Mother). That would amuse me.
Fifty years from now, when someone makes a cutting remark about her intelligence or schooling, she will still be muttering under her breath "I got into Northwestern, you know..."
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