[Dem0]

You need to realize the difference between CMS and blogging platform.
Wordpress.com is a blogging platform that allows you very little customization and the domain will be yourblog.wordpress.com
Wordpress.org CMS allows you to have a fully editable website/blog that you host on your own server with any domain you want.

If you just want a simple blog with basic functionality and no cost, than go with Wordpress.com, and someone correct me if I'm wrong but Drupal and joomla only serve as CMS and not as blogging platform.

[The RooT]

You can get a free ".wordpress.com" Sub-Domain or you can buy a Domain & download the wordpress software and set it up on your on Domain.

I personally prefer WordPress for Blogging Software.

Off Topic: If you do make a blog, please PM me the link or post it under Website Showcases because i would really love to visit your blog!

[NoScript]

Is it only WordPress that allows you to create your own blog, free of charge using their .wordpress.com extension?
You can also download the software and host it on your own domain.

The other two questions I can't answer because WordPress is the only blogging software I have messed with.