Here are a list of search qualifiers you can use for issues:

Search In

The in qualifier limits what fields are searched. With this qualifier you can restrict the search to just the title, body, comments, or any combination of these.

warning in:title
Matches issues with warning in their title
error in:title,body
Matches issues with error in their title or body
shipit in:comment
Matches issues mentioning :shipit: in their comments

Author

The author qualifier finds issues created by a certain user. For example:

cool author:gjtorikian
Matches issues with the word cool, created by @gjtorikian
bootstrap in:body author:mdo
Matches issues written by @mdo that contain the word bootstrap in the body

Assignee

Much like with author, the assignee qualifier finds issues that are assigned to a certain user. For example:

assignee:vmg
Matches issues assigned to @vmg
assignee:fjakobs repo:ajaxorg/ace
Matches issues assigned to @fjakobs within the @ajaxorg/ace repository

Mentions

The mentions qualifier finds issues that mention a certain user. For example:

resque mentions:defunkt
Matches issues with the word resque that mention @defunkt anywhere
mentions:azakus assignee:arv
Matches issues that mention @azakus but are assigned to @arv

State

You can choose to filter issues based on whether they're open or closed based on the state qualifier. For example:

terrible state:closed in:body
Matches closed issues with the word terrible in the body (how rude)
ie state:open mentions:fat
Matches open issues that mention @fat with the word ie

Labels

You can narrow your results by labels, using the label qualifier. Since issues can have multiple labels, you can list a separate qualifier for each issue. For example:

cat label:bug
Matches issues with the word cat that are also labelled with bug
label:bug label:resolved
Matches issues with the labels bug and resolved

Language

You can choose to search for issues within repositories that match a certain language with the language qualifiers. For example:

language:ruby state:open
Matches open issues that are in Ruby repositories
mentions:holman language:bash
Matches issues that mention @holman within repositories containing Bash code

Created and Last Updated

You can filter issues based on times of creation, or when they were last updated. For issue creation, you can use the created qualifier; to find out when an issue was last updated, you'll want to use the updated qualifier.

Both takes dates as its parameter, in the format of YYYY-MM-DD--that's year, followed by month, followed by day. You can continue to use < to refer to "before a date," and > as after a date. For example:

language:c# created:
Matches issues that are open for repositories written in C#
weird in:body updated:>=2013-02
Matches issues with the word weird in the body that were updated after February 2013
author:puzrin created:2012-11..2012-12
Matches issues created by @puzrin created between November and December of 2012

Comments

You can choose to qualify matched labels based on comments by adding a comments range. For example:

state:closed comments:>100
Matches closed issues with more than 100 comments
comments:500..1000
Matches issues with comments ranging from 500 to 1,000