The 'gay' comment wasn't really foreshadowing at all; it was just supposed to be a funny joke, and by coincidence, Willow turned actually gay. Of course now we've seen all the seasons it's foreshadowing, but I don't think we should look at it as "Okay...Willow just realized some homosexual potential...there goes her heterosexual relationship!", because I'm sure that's not what the writers had in mind.
About the human-vampire personality business, I agree with AS. To illustrate the point a bit further let's take a look at Spike and Darla.
Spike was a typical example of a mama's boy, no question. I think there are some traits of him that have been consistent as human and vampire, though: his emotionality. I mean, how evil Spike cried over Drusilla is probably something not every vampire would do, so it's clearly something that doesn't come with the vampire-package. However, the outing of that emotionality in human and vampire Spike is compeltely different. Human Spike dealt with it in his sissy poet ways, while vamp Spike got drunk and threatened to do violence. So there's the difference. The actual personality traits are still there, the demon inside just releases inner desires and viciousness that everyone to some level possesses.
Somebody like Harmony would of course not be very affected by this, she was always just that shallow, dumb girl who was already living at her fullest potential. On the other side we have Angel, Spike and Willow, each of them being repressed some way as humans, either by themselves or outer circumstances.
And then we have Darla. To be honest, we haven't really seen a lot of her back from her human days, but from what we have seen I think we can say she was not really that repressed. She was a cynical prostitue who didn't believe in God...not exactly the poster child for good behaviour back in those days. Consequently, I think she kept a lot of her human traits as a vampire. If I had to describe vampire Darla, the first words that come to mind are 'evil and seductive'. Of course, Darla was probably not super evil as a human, but her outcast status in society allowed her a lot more freedom to really be herself, which might have made her seem 'evil', well, or at least not like one of God's children, to others. So she, like Harmony, didn't turn into what seemed like a completely different persona upon being sired.
(Of course this whole Darla part is extremely hypothetical, as we've seen her originally human in exactly one scene...but I thought this makes sense anyway).
Another thing I just realized, the whole character thing might also be related to the power of the demon that made you. Spike, Angel and Darla all came from a very strong blood line, while Harmony got sired from just some random no name vamp. Doppelgangland Willow might have even been vamped by the Master himself, or someone else of the order of Aurelius, giving her very powerful, but also very evil blood.
