I am a crank WRT the semantic web. Sure, there will be pockets of success, but I think the whole concept is very 19th century. It is almost the opposite of the network effect; it is not of much use unless a significant portion of the sites on the web are marked up. Its whole search mechanism is logic based, while much out there in non-logical. There is all that W3C navel gazing to learn. Just about anything designed by a committee, in anticipation of a revolution, fails.
I think the semantic web will fail because more revolutionary technologies will pop up out of nowhere and surprise us all.
Alternatively, the pockets of success can be significant. I have zero hope of it being "search engine" worthy, since it's too easy to fake (see meta keyword).
But... in more controlled environments, it's a very powerful tool. Corporate networks and Medical Research are two areas where it's getting bigger, and has made significant advances.
I love the idea, but it's aimed wrong every time it's explained. Trust is a big issue, and there's no way to prevent lies. But in the absence of trust problems, it's incredibly powerful.
I think the semantic web will fail because more revolutionary technologies will pop up out of nowhere and surprise us all.