Email is the obvious one. With their email, you can get back in touch with them and pull them back into the site after they (inevitably) drift off.
Email is the bit that sticks... without email capture, there is no stickiness. Without stickiness, growth is extremely difficult. You would need to make a site so compelling that people come back to it even without being reminded. That's really, really hard (think "building the next google" kind of hard).
Totally agree, but if you are going to capture little bits of info say: Zip, Sex..etc From experience I was wondering which was most useful to increasing activity and membership and how people creatively leveraged them.
Well, personally I don't think that any of the other fields are worth capturing until you have started building a relationship with the user... then, it depends on what sort of information you want to send them. What job they have can be very useful (for example, if you're operating within the construction niche and your message to project managers is very different from your message to architects), but those are all things you grab in the form after the one that grabs email...
I think which elements are essential really depends on your site and what you're trying to communicate.
Email is the bit that sticks... without email capture, there is no stickiness. Without stickiness, growth is extremely difficult. You would need to make a site so compelling that people come back to it even without being reminded. That's really, really hard (think "building the next google" kind of hard).