By default, the rich text editor defaults to the "Filtered HTML" text format, which can be a pain if you're expecting to be able to add images and other media to your content, or if you want your text to be formatted using proper HTML.
There's an easy way to switch the default format: Change the order they are listed. The only reason the system defaults to Filtered HTML over Full HTML is because they're listed alphabetically.
To fix this, go to your administration menu and choose Configuration -> Content Authoring -> Text Formats.
Click on the crosshairs next to Full HTML and drag it to the top of the list, then click the Save Changes button.