Signal to noise ratio
it should not be a surprise that so many blogs are crap, and only add to the noise in the equation. most people's thoughts are not worth writing down, and even when such material is worth putting into print, most people suck at writing, as they suck at communicating coherently by any method.
try to imagine GW Bush's blog, sans speechwriters and handlers. go on. i'll wait for you to stop laughing...
the reason why good blogs are highly regarded (and very uncommon), is the same reason good authors are widely read (and likewise rare). those who thoughtfully consider what they wish to communicate, and then express it effectively and engagingly, are just as uncommon online as they are in literature.
mostly, the blogosphere is nothing more than mental masturbation: some twit unspeakably aroused by his or her own astuteness, bothering a keyboard and taking up bandwidth. such a waste. the world would be better served if all the blogging twits would just get out more (or at least strive to be more thoughtful, knowledgeable, articulate, and most importantly, relevant).