Actually, I was thinking about this on my way home from work. There are the obvious Christian answers, which are important, but kind of a given in this forum - at least I would hope they were.
For me, it's all about her approachability and openness. I don't think I would ask a lady out based on her looks alone ("Hey, you're pretty! Can I have your number?"). I would have to have some sort of interaction with her as well. Because of this, I see nothing wrong with asking out someone from my pool of friends. I am not a big believer in the whole "friend zone" thing, and I think most good friendships could handle an awkward question here and there. Besides, if the simple act of asking her out for a chance to get to know her better makes her so uncomfortable that she gets weirded out every time she sees me, then she wasn't that great a friend to begin with. Besides, she may be attracted to me as well, so...ya never know.
What I'm looking for from her once in the relationship (which I suspect is the heart of your question)?
I'm going to jump immediately to bad times. How is she when we disagree on something? Is she respectful? Does she call me names? Is it her way or the highway? Is she a pushover or a people-pleaser? passive-aggressive? Willing to compromise? Does she stop being affectionate if everything in her life is not perfect? Can she explain her view and try to understand mine as well? Her character in a disagreement speaks volumes. Most any woman can be pleasant during the good times. A woman who can be respectful and compassionate when things aren't going quite right is a rare gem.
supportive but not clingy - She is supportive of my hobbies and interests and sometimes participates with me. But she is her own person, and has interests of her own. I support her in these as well and participate in her interests on occasion. We have found a good balance and do not smother each other.
She is classy but unpretentious - She is equally comfortable with going to a symphony concert as she is getting on her #%^&-kicking boots and doing the Texas Two-Step.