The most important metrics involve what users do once they land on your site. From a web development perspective, that is the key to making sales.
One thing that always bugs me is that when website owners and developers attempt to fix problems of a site, the solution they gravitate to is more search engine friendly than user friendly
Search engines dont buy any products, or fill out lead forms, but people do. So designing a site that is search engine friendly should always come 2nd to designing a site that is user friendly. Every sites goal should be to please the end users first.
The goal of a search engine is to send traffic to a website that solves their visitors problem. Search engines are constantly monitoring, adjusting, and making improvements to their algorithms to find websites that are more user-friendly and more relevant for specific searches.
A great tool to see how a user interacts with your site is a heat map. A heat map tracks and shows where your users mouse clicked. There is a free version of the heat map technology at http://www.labsmedia.com/clickheat/index.html.
Crazyegg.com is the most popular service that offers an advanced version of the heat map technology providing results on a monthly basis.
Click Density also has a less expensive heat map generating service at http://www.clickdensity.com/.
The way the navigation of your site is set up is very important to increasing user friendliness and website interaction. Make sure that at all times users can go to your home page. A good practice is always to have your logo clickable to your home page as well.
Make sure that users can distinguish between clickable links and regular text. The new idea that links should be without underlines and look like regular text has been growing rapidly online. I can tell you that this is a big turn off and will decrease your visitors navigating through your website. The best color for a link is the well-recognized blue, with an underline that we are all used to from early Netscape Navigator days. That never fails.
Make sure to use a light color for the background with a contrasting color for the font. A perfect example is black letter on a white background. There is a reason why the major websites and portals online use black letter on white background; its a lot easier to read. It doesnt strain the eyes as white letters on a black background do. Make sure your design does not have any negative impact on the end user; eye strain is clearly a negative impact. Even if the user doesnt know it, they are subconsciously going to veer away from your site to a competitor that offers an easier read.
The visitor came to your site for a reason generally to solve a problem. Focus on that and make it easy to find solutions and obviously to read and understand them its that basic.