Consistency for websites can be achieved in a few ways. If one page is all dark blue using Times as a font and the next is bright red using Arial, most people would consider those pages as completely different and unrelated at first glance. From a psychologically aspect, people are more comfortable knowing where they are. Basically, the unknown scares people. On the web, good design helps reduce that anxiety by showing where that page is on the site relative to all other information.
Structure is also very important to any complex website.
Navigation through a site's overall architecture is essential to backtracking and finding broader ranges of information. If the data is just thrown together loosely with no cohesive structure, people would give up trying to find the information they want. Depth versus breadth studies have shown that sites with many levels using few links each are faster to navigate than few levels with many links each. The key to a good structured website then is to have the levels divided logically. It is also very important to have links to allow for backtracking within a level and between levels.