It's probably heresy here, but a lot of people still consider email a terrible way to get to know someone and online means in general to be extremely limited. An online dating site should be like Google: give me a bunch of options, and I'll make a quick low-bandwidth decision whether to invest in a face-to-face encounter. Engaging in an email conversation with someone isn't going to result in a better decision than making a snap judgment based on their picture and profile. I think it's more likely to result in a worse decision, actually. When I meet a potential date online, I just want to get face-to-face over drinks (coffee or alcohol) as quickly as possible so I can decide whether to have a real date with her.
For me, the only role of the initial contact is to say, "I would like to meet you. Would you like to meet me?" Of course, you also have to communicate that you're human and that you've actually read her profile, and you have to avoid communicating any desperation, narcissism, or other creepiness. So it's good to keep it short and make a little bit of conversation about something in her profile just to prove you read it.
Email is a good way to get to know someone on a technical level where you're kicking around programming problems or mathematical theorems. That's probably one reason why international open source collaboration works so well - people are (1) focused on the technical and (2) often share a common internationalized hacker/geek culture
On the other hand, to get to know someone in a romantic/sexual chemistry way, email is atrocious unless the people involved already share a lot of cultural reference points.
It's a lot like sending out a resume - you're not trying to pack everything about yourself into the written text, you're just trying to be interesting enough to get a phone call. With the phone call, you're trying to be interesting enough to get the low-risk face to face meeting. And so on...
For me, the only role of the initial contact is to say, "I would like to meet you. Would you like to meet me?" Of course, you also have to communicate that you're human and that you've actually read her profile, and you have to avoid communicating any desperation, narcissism, or other creepiness. So it's good to keep it short and make a little bit of conversation about something in her profile just to prove you read it.