I think you're quite right. There would be a lot of time and effort involved. And it seems to me like the volume of metadata about things would exceed the volume of things, like adding complete descriptive metadata to a haiku.
But could adding semantic tagging be simplified? Clicking a "like" button defines a relationship in a very simple way. The Delicious browser plugins are another way to create relationships and add tags. If things like that were standardized and made open and interoperable, then that would be a step on the way to the semantic web, would it not? If things like that were easy and intuitive and offered obvious value to people then the whole process of semantic tagging could be crowdsourced.
The computer has to do its part too. Doesn't Google track users' search histories? If they keep track of what I type in, and what I click on, then they can start to understand something about the way I use language, and map that to the labels and relationships that are already out there online.