I came across a few useful features in the WordPress.com FAQs while researching the domain upgrade/mapping functionality. One is related to embedding content (digg, videos) to a post, and another related to the filtering of the site feed.
WordPress.com has a pretty strict policy against scripts and objects in a blog, and due to this not many widgets can be embedded in a post or in the sidebar. However, certain items can be embedded by making use of special tags.
One of this is the digg widget, for which the post needs to be submitted on digg first to get the corresponding URL. I gave it a try using the most popular post on my site currently, and it seems to work as shown. Just add , where <URL> is the link obtained from digg. The only drawback seems to be that I had to digg my own post – not something I’d fancy doing on a regular basis.
There are also other tags that allow you to embed videos (youtube, Google video, kyte.tv), but I haven’t tried them out yet. However, I’ve seen many people embed videos on their WordPress.com blogs, so I suppose the feature works fine.
The Lifehacker feed has a feature using which you can subscribe only to certain tags, or exclude certain tags. WordPress.com offers a similar feature albeit only for category inclusion. For example, the feed corresponding to the bookmarks category for my blog can be accessed using http://abaditya.com/category/bookmarks/feed/. This can be useful when subscribers want to filter the content right at the feed level, though the support for the exclude feature would have been quite useful too.