It is incredibly important to focus your content to one field, niche, or industry and try to deviate from it as little as possible. The mark of a good blog is how consistently focused its content is, and a quick way to judge blogs in this respect is to look at their tag clouds (and if not available, just look at the list of categories).
The idea struck me when I was looking at a group of tag clouds on Pollster based on the MSNBC Democratic debate. The tag clouds analyze the speeches given by the participants and shows the most frequently used words in larger and bolder print. Look at the following three clouds:



As you can see, none of them present a clear train of thought (perhaps rightly so), rather they are cluttered with several disparate ideas. Applying the same idea to a blog, have a look at the tag cloud from my friend Stan Schroeder’s blog:

Right away you can see that Web 2.0 is the prevailing theme of the site, but that he also looks at the social web as a subset of the larger space. Focusing your content is the first step in building a loyal readership.