Comments support Markdown syntax, extended by some handy tags:

@426 links to post number 426
#Dragon_Ball links to tag “Dragon_Ball”
+Pirate links to user “Pirate”
~~new~~ adds strike-through
[spoiler]Lelouch survives[/spoiler] marks text as spoiler and hides it
[sjis](´・ω・`)[/sjis] adds SJIS art

You can also specify the size of embedded images like this: