All WordPress blogs by default have comments open on all Pages and Posts. You can enable and disable comments on free hosted WordPress.com blogs on this page > Settings > Discussion
Discussion settings is where you set up defaults for comments.
You can override the default settings and enable or disable future comments on any individual post or page.
Note: By default, new blogs on WordPress.com will have the Discussion hidden. You can reveal it by clicking on Screen Options http://en.support.wordpress.com/screen-options/ on the top right corner of your Admin page, and checking the box that says Discussion so the module is included. http://en.support.wordpress.com/modules/#list-of-modules
See also http://wpbtips.wordpress.com/2010/04/23/comments-off-1/ under "Faster way" and "Even faster"