I just wanted to put a quick note that QUERYTRACEON is now publically documented in http://support.microsoft.com/kb/2801413/en-us. If you have situations where you don’t want to enable an optimizer trace flag globally on your server instance, you can use this hint for a specific query. Note that we only support the trace flags listed in the KB article.
That is extremely cool. I'm torn between being angry at you people for not documenting this before and being happy that I know this exists. :-)
There should be a note in the documentation regarding permissions needed to use the hint.
In order to use querytraceon, app user should be a member of sysadmin role. It is not usefull for production. There are couple workarounds ( using stored procedure execute as owner or using plan guides ) but still not suitable for production.