The current web is heavily polluted with various forms of spam. Can semantic technologies help? If the spammers start using fake metadata then it could reduce the usefulness of what we currently have.
Could we make use of personal networks to assign reputation to sites and people? I've heard of people using their FOAF network as an email filter, i.e. only accept incoming email from people in their extended network. Google's social search can give you search results using network data in your profile that can be based on FOAF and XFM data.
This question was partly inspired by the classic Metacrap article.
I know I keep mentioning FOAF and XFN, but those are the semantic technologies which I have played with most.