Jakob Nielsen, the well-known usability guru, offers a list of the "Top Ten Design Mistakes" for weblogs. There's lots of good stuff here, but the basic item to remember is that weblogs are web sites. Even if you're using an authoring tool (Movable Type, WordPress, TypePad, etc.), it's incumbent on you as a weblog author to provide information on who you are, to stick to your topic, to publish regularly, and to construct meaningful links.