Yeah I think it was just because she was sassy and bad ass, like a young (modern) Darla kind of. I think a lot of people responded to that as she also actually seemed to be more of a threat to Buffy than any run of the mill vampire (at least at the start) and had the writers wanted to I'm sure she could have made a great villain throughout the season.
You know what - that would have been a far more interesting plotline then Adam! They could have utilised him too... for like an episode, but Sunday as a vampire slayer turned vampire would have been... wow, that had potential and real emotional effect on Buffy.
Ugh, I hated Sunday. There was basically no reason as to why she was so powerful, and I wish they'd at least given some explanation, like the Slayer-turned-vampire thing.
When was it stated that she was "so powerful"? She beat Buffy in the first fight because she psyched her out and had better moves than her. Was the vamp that stabbed her with her own stake in Fool for Love super powerful? No, he just got the best of her.
I liked Sunday...mainly because of her taste in clothes haha.
I haven't rewatched "The Freshman" since I remember how annoyed I was by Sunday, so I'm going by pure memory here... But the vamp in FFL was a totally different story--Buffy was clearly more powerful than him; he just got a lucky shot in. Buffy was comprehensively beaten by Sunday, completely thrashed in that first fight. (I admit, I do have to rewatch the episode, but that's what I remember anyway.)