MHO the welcome message is not sufficient for this. There should be just the most important functions there because the welcome message is usually read at most once and then the less important topics from it get forgotten.
Certainly the most important thing I am missing in Discourse is a formatting help. It should be easily accessible when the edit box is open. Something like on Stack Exchange:
The “advanced” button with a menu of the advanced formatting functions could be helpful too. Though I have the gear icon suggested by @codinghorror connected with configuration / advanced parameters and not other functions. As I noticed here in Discourse the gear icon is already being used for the user’s profile “Preferences”.