Jokes aside: microformats started to get pretty good traction, but the biggest challenge (as I see it) has always been adoption in software, rather than encoding the data itself.
Flock was the great white hope in this space for a browser which (somewhat sensibly) used the semantic web to enrich people's lives, but there wasn't really a killer app for it. It did a lot of interesting stuff, but none of it was omfg can't live without youuuuu.
If browser vendors start building great features to take advantage of the semantic web, then developers will start adopting and consumers will start [tacitly] demanding it.
Interesting point: if you go back to the SciAm article which kickstarted a lot of interest in the semantic web amongst relative laypeople, then you'll find that actually it's not dissimilar to where we are today, but we are getting there without the semantic web.