So you've finally convinced your friend to join the ranks but he/she needs some help learning the ropes, so to speak. The two things I would immediately share with my friend are:

The Cosmos UI suite of add-ons.  This is the grand-daddy add-on of all add-ons for WoW. Every serious player I know uses this collection of add-ons, and for good reason. The list of features it adds to the game is much too exhaustive to list here. It's regularly updated to work with patches that Blizzard rolls out, and is a truly essential tool.

Stuck in a quest? Not willing to pay for a strategy guide? Thottbot.com is the resource I turn to often to read how to finish a quest or find a particular NPC. Some would consider it cheating, but I don't. Particularly for a new user to WoW, it can help them get comfortable learning the game if they're not constantly running around in circles wasting time to figure out a quest.

What would you recommend to new players of WoW?