Buffy was sacrificing her life for Angel, I don't think 'joy' is the appropriate term here. She maybe should be relieved- but she clearly feels pleasure during the entire act. It's not just slayers- what sense would that make if it were a unique slayer thing?
We see other instances of the bite being pleasurable on the show (plus in almost all vampire mythos) but this was the first time it was explicitly shown.
Liam feels pleasure when Darla bites/turns him. William feels pleasure when Drusilla turns him (he also feels pain, so obviously there is still some pain even if the act is 'gentle')
Buffy also feels pleasure when Dracula bites her, so it wasn't just an Angel thing.
And they've even made it canon in the comics in a conversation between Angel and Willow in S9.
The vampire bite is a metaphor for a sex act, always has been. Mostly its a violation/penetration by a vampire. But there can be a more overt sexual/seductive aspect if the vampire chooses to be gentle, (like say turning the victim) but even if not there's a good reason why victims stop struggling as soon as their bitten.
The fear of victims seems mostly comes from the fact that a) the person attacking you has a deformed face/big teeth/snarling like an animal and b) that they're not being too gentle while their draining you dry.
I seriously doubt that Buffy actually thought initially that Angel would kill her. That he would have killed Faith, maybe, even probably, but she was counting on him to stop in time. But the look on Buffy's face after she kicked the lamp or whatever it was, was fear, not passion. Maybe she finally realized that what was feeding on her wasn't Angel, but the beast who she had awakened by striking him. Maybe she was flashing back to the last time that a vampire had bitten her, or maybe she just finally realized the gravity of her action, but, orgasm or no orgasm, when Angel actually started feeding, she was afraid. Maybe she finally realized that it wasn't her Angel who was draining the blood out of her, but the demon inside him, the demon she had awakened by striking him. Angelus.
Dracula has always been portrayed as having the power to enthrall his victims, and, yes, his bite is always a metaphor for seduction, but he has also always been portrayed as unique among all vampires, even going so far as to remain in human form when he bites his victims. No snarling beast with bared fangs there, so using him as an example of all vampire attacks doesn't actually work. On the other hand, Angelus' victims, the few that we ever saw, didn't seem to be that happy about being bitten, and that snarling beast that you mentioned was definitely present, just as it was when he bit Buffy. I would say that Angelus' feeding was more a metaphor for rape than seduction. As for Darla biting Liam, or Dru biting William, in neither case did the victim know on advance what was about to happen. Buffy did know, or at least knew what might possibly happen, and she said at one point in the show that being turned was one of her greatest nightmares. Maybe Angel wouldn't ever turn her, but the beast who she had awakened quite possibly would, before Angel could bring it under control again.