I've done some collision analysis of the wikis at WikiPopularity. The results are at [Wiki Structural Markup Usage] and [Wiki Character Markup Usage]. (I've split this into two pages so that the tables display across the width of a single page.)

The analysis shows:

# the choice of **..** and //..// for bold and italic is collision free
# {{..}} for images collides with five wikis:
## MediaWiki for templates
## Confluence for mono
## [JSP Wiki] for mono
## Wikispaces for mono
## StikiPad for mono
# {{{..}}} collides with MediaWiki template parameters
# == for headings has two partial collisions
## TikiWiki where ===..=== is used for underline
## DokuWiki where the heading order is reveresed, that is ===== is a level 1 heading
# [[link| text]] has a partial collision with TiddlyWiki
# *.. (bulleted list) has a collision with PukiWiki where it is used for heading
# #.. (numbered list) has a collision with Midgard Wiki where it is used for heading
# <<..>> has a collision with TiddlyWiki, where it is used for macros