I think that has a lot to do with the characters rather than the couple. People love Spike, people love Wesley...both men were in love with women who didn't love them and so that added tragedy and romance to the relationships that maybe wouldn't have been there if the characters weren't so popular. It made them more attractive to the viewer.
Possibly, although it might not be "attractive" so much as "relatable". With Spike, I kind of... ached for him. I wholeheartedly believe that he loved her, but I also believe that his love brought him misery because he really wasn't meant to feel that way. He tried tried to stop himself, but he could never resist his impulse to be near her and help her (favorite example of this: I'm free if that bitch dies! I better help her out). Although, yes, in the end I did like him more.
There was a Fred/Gunn relationship? Man, it's been too long since I've watched Angel. I liked Fred/Wesley, though, from what I remember.