Having been doing Internet marketing and developing websites that rely heavily on natural search engine traffic for the past few years I can say without a doubt the largest challenge that site owner’s face is getting traffic into their website and that doesn’t include getting highly targeted traffic which is looking for your content. "Build and they will come" is nothing more than a slogan on your car bumper sticker.
The second largest challenge, of course, is converting that traffic into your desired result. That could be newsletter signup, sale, new reader, blog comment, new StumbleUpon fans or Twitter followers, or even just a new friends on your facebook profile.
Key To Success: Multiple Traffic Sources