Well, all this talk of protective heroes makes me want to talk about danger.
The edge in romance, the edge that makes it erotic, I think is danger-- not necessarily danger from the outside, but danger from the romantic character. That is, falling in love should be like playing with fire somehow. It's not just a nice peaceful benevolent experience, but somehow dangerous to the self. It can END up being all supportive, but if it's that way all the way through the book, if love is always safe, it's likely to be boring because there's no conflict.
What is the danger of love? Well, the first is always-- you must give up yourself or part of yourself to love someone. No arguments, please. It's true. Sure, you gain too, etc. But when you are standing on that cliff, looking at love, jumping is scary. Why? Because you might lose yourself.
You can't know, when you start to fall in love, that this is going to be wonderful. Maybe it won't. Maybe it'll lead to murder. Maybe you'll give up your desire to become a singer so that you can follow him on his mission trip to Rwanda. Maybe you'll be swallowed up by her intrusive family. Maybe his hazardous lifestyle will keep you in a constant state of terror. Maybe your values are opposed and it means you give up what you really want-- children, adventure, ambition, a job, your closeness with friends, I don't know-- in order to be with him.
Love is scary. And it's scary even if this is going to turn out to be The One. In fact, it might be more scary then, because there's no turning back. You're probably going to end up caring for this person more than you care for yourself... and that's really scary.
Anyway, while love can be supportive and might end up enhancing your life, if it seems that way all the way through the book (all together now), there won't be much romantic conflict. Love really is dangerous-- this isn't something made up by romance writers-- and (this is really important), it is dangerous even when or especially when it's the Right Love. "Loving is giving hostage to fortune," remember? When your happiness is dependent on another, you can be unhappy a whole lot.
So... how can a potential lover be dangerous? If you have a romantic relationship in your story, what would you say is the danger posed by one to the other?
Denny and I were talking about this, and I said I thought her hero's danger to the heroine was specifically that he held the key to the past she doesn't want to remember. If they become intimate, he might inexorably lead her to discoveries about that past-- bad stuff.
Buffy and Spike-- well, her danger to him is that she makes him want to be good, and that makes him feel bad. (Well, he IS bad, but he's never felt bad about it before. :) She is dangerous to him because she tempts him into giving up what he thinks is fun, great, all that.
His danger to her is more subtle. He loves all of her and doesn't see The Slayer as separate from the woman. She wants to believe that this killing machine part of her isn't really PART of her, that she -- the real her-- is still innocent within her, and will still be there when she's done being the slayer, that slaying is just a job. From the very start, Spike sees her as the slayer, and loves her as the woman who is the slayer. To accept his love would mean she has to love the part of her that she secretly hates.
Darcy and Elizabeth, since Theresa brought them up. Darcy is dangerous to Elizabeth because he disapproves of her family and is a constant reminder that she can leave her family. She wants to love her family in a pretty uncomplicated way and not see their faults. Darcy is partly wrong about the family-- he IS a snob-- but he's partly right too, and she really doesn't want to see the family plainly.
Elizabeth is dangerous to him because she continually taunts him into a loss of control. He is quite controlled and thinks he has to be as a young nobleman of good intent. He can't give into his anger and passion, so he has always taken refuge in his hyperresponsibility and his class-oriented repression. As a not-quite-respectable miss, she taunts him into realizing what he has given up for the repression-- and teases him into uncontrolled displays of emotion. She makes him FEEL, and feeling is dangerous to him as it is uncontrolled.
Other examples? Your own?