I wanted to make images float left or right, so I thought it would be nice to be able to pass a class attribute through the img plugin to the final img tag.
An example of the feature in use can be seen here (notice class="floatleft" and class="floatright"):
http://www.cworth.org/
And here's a patch to implement it. Will this survive markdown munging? It seems quite unlikely... How does one protect a block like this? Oh well, we'll see what happens.