On pages where more than one post is listed (eg posts of a given tag or author), it is very hard to determine where one posts ends and another begins. This is partly related to the very similar heading styles of different sizes.
I think the following things could improve the l&f:
Have the title of a post in a different color (when displaying a single post the title is for example greyish, but this is not the case if there are more than one posts on the page)
Use a divider lines of sorts between posts