Buy hosting and install wordpress. Simple.
WordPress.COM is a multi-user platform which means that we all share the same underlying theme and wordpress files. Any security hole or bad code that would be introduced by one blogger, if they allowed plugins or javascript or flash, or whatever, could possibly take down the entire system.
You would be the first person back here in the forums yelling at the top of your lungs if another blogger brought in some bad code that brought down your blog.
Facebook, Myspace, Blogger, etc., all fight that all the time, and they have all had times when it has brought down parts of their systems. I make a good bit of money fixing blogs killed by bad code in a plugin or some fancy javascript with dancing bunnies. Recently I cleaned and repaired 5 sites for one client that was caused by a bad plugin.
Get hosting and then you can do whatever you want. If you install a bad plugin or code (and there are a lot of them out there) that kills your blog, then you don't bring down everyone else.
Also, watch out for themes. There is an increasing number of themes out there with malware built right into them.