You're considerably limited as far as customisation goes if you go for a Wordpress.com blog.

IMO if you want a hosted solution go with Tumblr, if you want a self-hosted solution go with WordPress (www.wordpress.org, not .com).

You can customise a Tumblr blog quite a lot and you have the added benefit of not needing to worry about bandwidth (should you strike it lucky and land on the first page of Digg). For greater customisation though (i.e. do basically whatever you want) you should go with WordPress (by far the most popular blogging solution on the Internet).