Which is good? WordPress.com or WordPress.org? Both have their pros and cons.
A WordPress.com blog gets world class free hosting. So your blog is always fast. A WordPress.org blog in own domain, if not maintained well, may become slow. Also, some hosts have problems with some of the WordPress.org features. So, you need to choose your host carefully.
In WordPress.com you need not worry about anything except writing. But, in WordPress.org you need to have mimimum technical skills atleast. Or, you need to find someone who can maintain it for free or cost.
WordPress.com blogs are safe from hacking, viral attacks. In WordPress.org you need to make some timely measures to safeguard.
WordPress.com has a huge and active community and a portal for each language like http://ta.wordpress.com . So, even a newbie blogger can get good exposure and traffic for his sites.
A WordPress.org blog in own domain is best for branding. Period.
You can get many of the features, freedom in WordPress.org (like own domain name, theme customization) even in WordPress.com for a fee. But, the fee is too much. A WordPress.org site is cheap and best.
You can place Adsense and numerous other ads and earn money. WordPress.com is very strict against ads and blogs for money.
You are free to add as many themes, plugins and do whatever you want with WordPress.org . You can add third party scripts. Only when you explore these, you can realize the full potential of WordPress. It will really set your blog in to a power mode. A technically inclined blogger will really feel his hands tied in WordPress.com
Not Just Blogging:
WordPress is fast becoming a CMS (Content management system) of choice. You can create so many other things with WordPress.org . Not just blogging.You can make a membership only site, an ecommerce site, a twitter like site… Your creativity is the limit.
If it is your own domain and host, you can use it for anything like forum, file storage etc., You can install other software. But if you are with WordPress.com even in domain, this freedom is locked.
If you write many blogs, you can put them all under one domain. Adds value to your domain and it becomes like a portal for all your online activities.