<p>When designing a wiki application, what things should I take into consideration?</p>

<p>So far I have:</p>

<ol>
<li>revisions</li>
<li>parsing some sort of markup text</li>
<li>keeping track of links in wikis, and pages that link to other pages.</li>
<li>related wikis i.e. wikis are related to others.</li>
</ol>

<p>What else goes into building a Wiki?</p>

User interface -- one of the most frustrating things for users is that they have to learn one user interface for MediaWiki, one for TikiWiki, and one for any of the other myriad of wiki's out there.
The most important part of wikis is not the technical feasibility -- it's getting users to contribute and edit in a convenient and effective way. You can have the most technically robust wiki in the world, but if it's not easy to use, it will be useless as the community tool that a wiki is supposed to be.
Either copy an existing and familiar wiki syntax (such as MediaWiki), or be prepared to invest heavily in creating a WYSIWYG editor.