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The most common text element, used in almost all texts around the Internet, is a ParagraphOfText. It usually features words, of which some may require TextEmphasis.
As wikis are hypertext environments, they will usually need a way to mark InternalLinks and, as they do not exist in vacuum, also ExternalLinks.
It's often helpful to structure the pages using HeadingText and HorizontalLines, as well as NumberedLists and BulletLists, even if their overuse may be considered BadStyle.
For technical reasons, it's good to have a way to EscapeMarkup, or even include whole blocks of PreformattedText.
Both articles and discussions often benefit from IncludedFigures, both in form of images, drawings, photographs, and in form of mathematical equations, diagrams, charts and tables, maps, etc.
In science and engineering, superscript and subscript is very common. It is semantic, not a formatting issue.
Many articles refer to external materials, either by just mentioning them (often in footnotes or bibliography), or by using QuotedText. Both need to specify the source.