IMHO, there are 3 things that differentiate blogs from the Geocities/AngelFire/Excite/Xoom or NNTP days. The permalink: every piece of content has a unique URL. This enables RSS. Finally every blog has its own namespace. These three things will be the major reasons why RSS doesn't go the way of NNTP and Usenet and become irrelevant due to an overly high amount of spam. Using links and URLs (e.g. filter out the thousands of SPAM blogs at blogspot.com), we can filter out the noise; this was simply impossible with Usenet which didn't have links and separate namespaces.

BTW, this What is Web 2.0 essay from O'Reilly is magnificent dare I say a brilliant synthesis. Read the whole thing.

From O'Reilly Network: What Is Web 2.0.:

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One of the most highly touted features of the Web 2.0 era is the rise of blogging. Personal home pages have been around since the early days of the web, and the personal diary and daily opinion column around much longer than that, so just what is the fuss all about?

At its most basic, a blog is just a personal home page in diary format. But as Rich Skrenta notes, the chronological organization of a blog "seems like a trivial difference, but it drives an entirely different delivery, advertising and value chain."

One of the things that has made a difference is a technology called RSS. RSS is the most significant advance in the fundamental architecture of the web since early hackers realized that CGI could be used to create database-backed websites. RSS allows someone to link not just to a page, but to subscribe to it, with notification every time that page changes. Skrenta calls this "the incremental web." Others call it the "live web".

Now, of course, "dynamic websites" (i.e., database-backed sites with dynamically generated content) replaced static web pages well over ten years ago. What's dynamic about the live web are not just the pages, but the links. A link to a weblog is expected to point to a perennially changing page, with "permalinks" for any individual entry, and notification for each change. An RSS feed is thus a much stronger link than, say a bookmark or a link to a single page.

RSS also means that the web browser is not the only means of viewing a web page. While some RSS aggregators, such as Bloglines, are web-based, others are desktop clients, and still others allow users of portable devices to subscribe to constantly updated content.

RSS is now being used to push not just notices of new blog entries, but also all kinds of data updates, including stock quotes, weather data, and photo availability. This use is actually a return to one of its roots: RSS was born in 1997 out of the confluence of Dave Winer's "Really Simple Syndication" technology, used to push out blog updates, and Netscape's "Rich Site Summary", which allowed users to create custom Netscape home pages with regularly updated data flows. Netscape lost interest, and the technology was carried forward by blogging pioneer Userland, Winer's company. In the current crop of applications, we see, though, the heritage of both parents.

But RSS is only part of what makes a weblog different from an ordinary web page. Tom Coates remarks on the significance of the permalink:

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