Colons are widely misused to create block quotations in articles (they become semantically-silly definition-list definitions).
Wikitext badly needs support for the HTML blockquote element.
Textile text renderer uses "bq. " to render blockquotes [http://textism.com/tools/textile/]. Example:
bq. This is a block quotation.
Markdown text renderer uses email-style greater-than signs [http://daringfireball.net/projects/markdown/
> This is a block quotation.
Some combination of quotation marks may also be a convenient mnemonic:
"" Block quotation indicated by two double quotation marks at the start of the line.