Search engines credit established sites that have existed for some time. Their algorithms can tell if a site and its content is stable and consistent. A site's content can change and navigation may be moved around but the fact a site is consistent over time is important.
When a site is initially released, Google and other engines will often place a site in limbo for weeks or months before it assigns it a page rank. Pages may even be indexed but they will be considered temporary in the overall site value until they have aged and matured.
Social communities are examples of this aging factor. Individual editors can write content and see it live on Google days later. However, their blog, profile, or even individual pages won't receive a page rank for weeks to months. Google may see a site or community as an established website and deem it as creditible but not necessarily sections or individual pages of the site.
Google may index your individual website and display the pages in search engine results but when it comes down to overall site credibility engines may not trust the site as a valid source for some time.
Some website changes are considered healthy. Addition of relevant content and links is typically a good thing. Linking to an unknown website can have a negative impact. One key is to settle on a website page structure and refrain from altering a domain name or URL once the site is established. Once you choose a domain name to go with be sure to stick with it. Changing can kill your site popularity and credibility leaving you at square one.
Just like with encyclopedias or other sources considered credible, your website should remain consistent and should follow certain standards so search engines and readers alike value the content as credible and authoritative.