The answer is simply the way it is built. The purpose of a web site is to deliver information, offer utility, provide networking or sell some products. A web site should execute the task easily with out any difficulty to the user. It should follow the salient features and guidelines of web designing laid out. It should be user friendly with excellent internal architecture. Is the web site unique does it contain the right elements for the search engines to read and understand the site for proper indexing?
The web page design should be simple in coding and server side scripts should be used. Frame and Flas have to be avoided although the latter can be used in headers. The down load speed counts a lot in Seo hence you have keep the weight of images in mind.
Metadata and Title are very important although you can fine tune them often but making few changes is better. Always keep in mind the targetted search terms when creating the title and meta tags.