We don't allow plugins on WordPress.com for security reasons. Plugins are a feature only available for self-hosted blogs provided through WordPress.org. You can find out more about this topic here: http://support.wordpress.com/plugins/
For more information on the differences between WordPress.com and WordPress.org, please see: http://support.wordpress.com/com-vs-org/
As for themes, they can't be uploaded to WordPress.com. That's a feature of self-hosted blogs through WordPress.org.
However, we offer over 200 themes, both free themes and premium themes created by leading theme companies. You can view all of our themes at http://theme.wordpress.com/
You can also post suggestions for new themes we might add on our public user forum: