MediaWiki has its own markup, they call it wiki markup. This is the first lightweight markup I had experience with (if you do not consider PHPBB markup to be lightweight).

• https://www.mediawiki.org/wiki/Help:Formatting
• https://en.wikipedia.org/wiki/Help:Wikitext

Why I don't like it

• Syntax for bold and italic. ''italic'' and '''bold'''. This is classic! This syntax comes from very first wikis. But it's bad:

  • Easy to confuse '' and " when editing in proportional font. This alone is enough.

  • Six characters in total for one of the most used emphases (bold) is kinda uh.

  • Not available on the standard Russian keyboard layout.

• Headings have to be closed at the end:

  == heading ==

  And they also haven't resolved the level 1 problem.

• Indentation-based preformatted text.

• Inclusion of HTML tags such as <br> and pseudo-HTML tags such as <nowiki>.

• Support of inline styling. Sure, because of that there are Mediawiki wikis with really good visuals, but you know the problems.

• Ridiculous linking of namespaces File and Category.

• Two different syntaces for local links and external links:

  [https://example.org text after space]
  [[article | text after bar]]

• List items are limited to one paragraph.

• The syntax for image insertion is the same as linking a text page, except it's different.

• Tables that are so strong they replace everything not supported by the markup.

• Magic words like NOTOC and #REDIRECT.

• Wrong line-break logic.

And for some reason, on mature MediaWiki wikis, almost every page is filled with countless {{}}s.

Why I like it

Still much better than Markdown.

• Bracketed links.

• Character-repetition-based list nesting.

• = for headings.

• Tables are useful.

• The colon element is cute.

• Good macros (called templates).