Multiple solutions were proposed how to extend Wiki systems with the ability to manage semantic annotation.
One which managed it to production state is the Semantic MediaWiki http://semantic-mediawiki.org/
One dedicated wiki-system employing SMW being
http://semanticweb.org/
SMW is a free extension to the widely used (standard) MediaWiki system. It strikes by it's simplicity.
The basic concepts of semantic modelling (RDF) are mapped to elements of the wiki system as follows:
Together forming one RDF-Statement.
The only real (and very intuitive) addition (change in syntax) being the Named Link allowing to describe the kind of relation between the subject and the object, the predicate name being constrained by a controlled vocabulary.
Thus there is no need, for new input-elements or significant changes in the editing syntax.
One drawback is the fact, that on one page only statements with the topic of the Page being the Subject can be constructed. But eliminating this constraint, while probably not much too complicated on the editing side, would impose a serious complexity problems both for operation on the data and human understanding of the data structure, thus leaving us with this trade-off between expressivity and complexity.