One other point that Jennifer made, and one that I think deserved its own post, was that if someone is writing a review for someone's magazine or professional website, that person's writing has been approved by a third party (with the writer and the reader being the first two), and that separates it from a person who just goes to blogger or livejournal and throws their own work up.
Obviously the issue of third-party approval has always been huge in the writing world, and it still is. If it wasn't, I wouldn't be pounding the metaphorical pavement trying to get some major publishing house to look at my novel manuscripts; I would just go to xlibris or some other pod publisher, and sell my books online. But readers, myself included, are more likely to be interested in something when someone has essentially said "I'm willing to invest in the cost of ink and binding on this, and ship it to Barnes & Nobles all over the country." Even with a professional website, if someone is getting advertising dollars, they are using space on something that they could be using for something else, and paying for the downloads, and that is its own kind of investment, though nothing like printing it.
It's true that the barrier to third-party approval isn't nearly as high as it used to be. Printing costs are far cheaper than they were in the pre-digital age, and the costs of putting up a website are so low nearly anyone can do it. Now it's more about distribution and the cost of eyeballs, but many of the same principles are still there.
I would argue in the case of blogs, however, the difference is that when you are reading blogs, you are your own acquisitions editor. In other words, there is no qualified person out there figuring out for you what's worth reading. You have to figure it out for yourself. That doesn't mean that the good stuff isn't out there, you just have to figure out what it is. That's probably why reading blogs takes so much time. So maybe Sturgeon's law has to be modified a bit regarding blogs, and more like 99.95% of everything in the blogosphere is crap as opposed to the usual 95%, meaning you have to work a little harder.
But, as the song goes, Whadya want for nuthin'?