A number of blogs have been linking to Laura's reflection on how blogging has changed in the past six years. I don't think there has actually been much decline in quality overall -- when it declines in one part it improves in another. But certainly a lot of things have shifted. I think Laura's probably right that people have localized -- blogger used to range far more widely, but now mostly stick to comfortable neighborhoods. I think regular use of feed readers has changed things; it helps people to organize their reading more efficiently, but by the same token means that they focus on things they already know they'll be interested in, leaving less room for the occasional delightful surprise. It's also been a major factor in the decline of the blogroll. I think the link monitoring thing has made a major difference; the TTLB Ecosystem used to be very useful, and then broke, and has never really been brought back to its former usability. Technorati used to be extraordinarily reliable; it's still useful, and still pretty much the handiest thing in this regard, but now it does weird things at regular intervals and is often very slow in listing links.
And things shift internally, too. Many excellent blogs have been quiet for years, or vanished entirely -- it seems like my blogroll has had a high attrition rate over the years, as I seem to have a taste for the excellent blog that lasts only a year. Most bloggers don't retain quality over long periods of time; it takes discipline and time, and people go through long spells of low quality. (I remember when, however much one disagreed with Myers, Pharyngula was a real science blog, with some very nicely written science posts here and there, rather than a blog devoted mostly to open threads, repetitious claims about religion, geeky adolescent jokes about sex and geekier but less adolescent jokes about squid, and bored trolling for online polls to crash. All those things used to be there, of course, in some form; it's the more interesting posts that have become much, much more rare.) Blogging isn't a wholly independent activity: sometimes people really do have more important things to do, and sometimes those more important things crowd out blogging itself. That's life. You can tell when I'm busy (or at least away from my computer much more than usual) because for weeks poetry, quotations, and links will be the primary kinds of posts; I do them even when I'm not busy, because they build up fairly steadily (the poetry less so than the other two), but when I am busy, they are the sorts of things that keep coming through the pipeline. They're easy to blog, and the material for them builds on its own. Blogging is not a very stable thing to begin with; it's not surprising that it changes.
Oh, and one obvious sign of shift: memes are much less common. There was a period where I was tagged for a meme at least once a week. Internet quizzes, book lists, ice-breaker-like things-about-your-self games -- they were extraordinarily common. But you hardly see them anymore. They never really contributed much to quality or content, but they did make blogging in general much more playful, and they did have the nice function of allowing you the opportunity to link to nice or interesting people who might not otherwise have been linked. They were the harmless neighborly chit-chat of the blogosphere, like talking about the weather with the neighbor two doors down, the one whom you like but with whom you have little in common and whom you rarely see. The blogosphere still has a playful streak running through it, but it's not so obvious anymore.
I think many things have remained constant, though. The majority of political blogs are still poisonous parasites or mindless bottom-feeders; and they are still widely read because it's hard to recognize the poisonous parasites who agree with you. The most interesting blogs are still usually those where people write in order to think things through, although some blogs are interesting more as enjoyable curiosities or as insights into other types of life or career (I have no interest in fashion whatsoever and yet I can spend a lot of time looking at the street fashion pictures at The Sartorialist -- so many people who are interesting in so many different ways). There are still trolls in comments threads, with some blogs attracting them more than others. Blogs are still a great way to discover and touch minds with people you would never have met otherwise. And for all the folly and malice, all the silliness and foible, all the sulking and vindictiveness, that can be found in the blogosphere, the level of discussion is still extraordinarily high for such free and open interaction.