These should all get escaped:

Backslash: \

Backtick: `

Asterisk: *

Underscore: _

Left brace: {

Right brace: }

Left bracket: [

Right bracket: ]

Left paren: (

Right paren: )

Greater-than: >

Hash: #

Period: .

Bang: !

Plus: +

Minus: -

These should not, because they occur within a code block:

    Backslash: \\

    Backtick: \`

    Asterisk: \*

    Underscore: \_

    Left brace: \{

    Right brace: \}

    Left bracket: \[

    Right bracket: \]

    Left paren: \(

    Right paren: \)

    Greater-than: \>

    Hash: \#

    Period: \.

    Bang: \!

    Plus: \+

    Minus: \-

Nor should these, which occur in code spans:

Backslash: `\\`

Backtick: `\``

Asterisk: `\*`

Underscore: `\_`

Left brace: `\{`

Right brace: `\}`

Left bracket: `\[`

Right bracket: `\]`

Left paren: `\(`

Right paren: `\)`

Greater-than: `\>`

Hash: `\#`

Period: `\.`

Bang: `\!`

Plus: `\+`

Minus: `\-`