Purple_martin, I agree wholeheartedly with everything you just said. I can't see how allowing authors to define their own tags is anything but a step backwards from standardization. My only concern is this...
Unfortunately, I think the discussion here indicates that everyone doesn't know what each tag means, since we've got several definitions going, all with perfectly valid reasons behind them, for one of the most basic and ubiquitous tags around...the plain <p>. Is it a unit of typography, or a unit of meaning? It was alluded above by bedlam that the two are the same, but I disagree. The typography of a paragraph is that is has a hard return on either side and (usually) an indent. But the decision of where to start a new paragraph and where not to is almost always based on it's role as a unit of meaning. I've finished pursuing a particular thread of thought, so I break to a new paragraph. That a double line break and a 3em text indent happens to accompany it is by-the-way.
rjohara, your post deals directly with this and brings the cryptic information in the Semantic Web stuff right into perspective. When you say that controlled vocabularies will be defined, I take this to mean within various areas of study? Is the intent that such vocabularies be stored in a central location, like the DOCTYPE definitions at the W3, or will fields of study establish their own local locations for these things? I'm facinated by the possibilities that this sort of collaborative effort could produce, if not simultaneously skeptical about its success.
The word "meme" was a mistype. Meaning units in language start with morphemes - word parts used to convey meaning, such as prefixes, suffixes, and root words - not memes. A meme is something else, entirely. Sorry about that.