Tangor is right, Zion. Imagine that one built a great new highway, complete with nice landscaping, rest stops with good food and cheap gas, a few toll booths, and so on. You still need a destination that people want to get to in order to create traffic. For a community, content is the destination.
Some specific kinds of content entice lurkers into registering and posting. Controversial topics are one. Polls are another, particularly if they are hot-button items (e.g., Windows vs. Mac, is the iPhone overrated, etc.) In some communities, "introduce yourself" topics will work.