Drawbacks to using pages for your main content:
First off I suggest you familiarize yourself with the differences between pages and posts (if you have not already): http://en.support.wordpress.com/post-vs-page/ .
For some types of sites, pages may actually be the best way of presenting the material, but wordpress.COM is aimed at blogging and using pages for your content has drawbacks you need to be aware of:
1. Pages will show up lower in search engine results because search engines place lower emphasis on pages. Pages are meant for static content.
2. When you publish a post, wordpress automatically "pings" all the major search engines to let them know you have published a new post so that they can index it. When you publish a page, wordpress does NOT "ping" the search engines. You will have to ping manually at a service such as pingomatic.com or wait for the search engines to find the page on their own when they make a scheduled crawl of your site.
3. Only posts show up on an RSS feed, pages do not. That means people cannot subscribe to your blog so that they are alerted when you add something new.
4. You can assign categories and tags to posts to help the search engines properly place you in search results. Pages cannot have tags or categories as they sit outside the blog structure which means your stuff will also not show up in the wordpress.COM global tags pages. For most people the global tags pages are responsible for bringing a lot of visitors to their blogs.