I think Drupal 7 stands a chance of being a killer semantic app, it’s the next version of a CMS with a sizeable install base and when the release is final and everyone starts upgrading suddenly tons of sites will be automatically exposing RDF in the form of embedded RDFa without any intervention on the part of users.
From a business viewpoint I think things linked data stuff like GoodRelations are a potential killer app, for example see these Scott Brinker and Priyank Mohan blog posts which discusses how BestBuy experienced a 30% increase in traffic after starting embedding GoodRelations based linked data as RDFa in their web pages.
In general linked data is an excellent way to augment your site with machine readable data and shows remarkable results in the visibility of your site, I’ve recently seen a couple of presentations by guys from the BBC whose wildlife finder application (a linked data application using BBC natural history resources) already outperforms some Wikipedia pages for animals in Google search results