There are some pretty exciting changes to wikis in SharePoint 2010. They are much more integrated into the publishing platform, and have page layouts, master pages, managed metadata and more with them. It eliminates the classic choice we always had with MOSS — is this a site or a wiki? The integration into sites is much more intuitive now.
You can also add Office content much easier into wiki pages, so there is no need to export a file as an image, upload it to SharePoint, then hook it up on the wiki page. Merging the structured data such as a list into the unstructured wiki will be way easier too.
One neat little thing: you can add any Web Part to the wiki page, such as a calendar say, and when you click on the web part the ribbon changes to help you add items to the calendar, remove items, etc. The contextual ribbon UI is really going to help streamline end-user tasks without all the navigating around.