Explicit is better, but you can tell a lot about someone by tracking their page views and click throughs. I remember seeing an article/presentation, "How to tell I'm a girl", but I cant find the reference. Some of it is pretty simple. If they are reading the sports page, they might be interested in sports. There is a bit of a problem with single computer households, but not huge.
The main thing is get engagement first; ask for the registration later.
The main thing is get engagement first; ask for the registration later.