Modeling the real world, approximations, and heuristics
As with traditional computer software, the Semantic Web provides capabilities to model the real world. The Semantic Web does in fact provider a much richer set of modeling capabilities than have traditionally been available to software developers, but it is still rather difficult to model many aspects of the real world, especially human institutions, culture, and social structures with anything better than a relatively low level of fidelity. That is not so much a negative statement about the architects of the Semantic Web as a very positive statement of the vibrant richness of human life.
At its heart, any model of any portion of the real world is simply an approximation of what exists and transpires in the real world. The Semantic Web does give us better tools for enhancing the fidelity of that approximation, but does not eliminate the semantic gap between the the approximation of the model and the actuality of the real world.
Many users find computers frustrating because on the one hand they have such promise and such immense capabilities and seem to do so well at many things, but then fail at even a lot of simple tasks. Web search is a great example. Half the time our favorite search engine actually seems to have read our mind and gives us exactly what we want with only minimal input, and then the other half of the time the search engine is completely unable to satisfy our queries no matter how hard we try. Why this dichotomy? The answer, in one word: heuristics.
Heuristics are techniques that computer software designers use as shortcuts to approximate a significant fraction of the "right" answer to a problem. The beauty of a great heuristic, like the beauty of any great shortcut is that it gets us to where we want to go with minimal effort. The downside of a heuristic, like the ugliness of any shortcut is that they do not always work or work as well as we would like and don't always help us get to all destinations that we seek. A great search engine employs a vast library of heuristics, but such libraries are finite. The great mystery is not that search engines fail us so frequently, but that they work as well and as often as they do.
Alas, the Semantic Web is not a magic bullet that will solve all semantic issues between computers and people, but it is a framework for modeling the real world and using heuristics to increase the fidelity of our software approximation of the real world.
The Semantic Web does have a lot of great promise to advance the state of affairs in how we model the real world, but we do have to remain cognizant of the fact that we will have to continuously "mind the gap" between reality and models of reality.