What do they want?
Who are your site visitors likely to be, and what are they likely to be coming to your web site for? They're coming on a 'quest'; what is it? If you can figure this out, and create site content that fulfills your site visitor's quest, you have a chance of keeping them on the site long enough to realize what a great product you have. If you fail to figure this out you are probably wasting your time with your web site. It's all about 'relevance' these days. We're way beyond the idea that people "surf" the web looking for interesting and beautiful web sites. Today we know that most web traffic is mediated by 'search', and most search driven traffic is driven by an information 'quest'. Match the quest with the content of the site they land on after a Google search, and you have relevance... bingo!
As a part of a larger communications plan, an effective site has a specific job. (Notice here that I didn't say simple. Simple and specific are different things. Simple isn't always good for a site -- specific is.) The more specific the job, the more effective the site can be. On top of that, there's a bonus. The more specific the job for the site, the more creative your creatives can get with it. Specificity has always been a critical component of great traditional creative. Not only is that also true on the Web, it's magnified. Specific goals lead to better creative. Better creative leads to deeper engagement, which further boosts effectiveness.
