Interact with target. How many people actually use this. Its starting to become more popular, especially with bot developers. First off you need to press escape and go into key bindings. Look for "Interact with target" and bind it to an easily accessible key (º for me, left of number 1).
Right click to move might also have to be enabled for some of these. Here are ways you can put this to use:
Grinding, target + loot faster - yep, sounds silly, but I'm sure farmers will appreciate not having to use the mouse to right click the target (move to him) and right click again to loot.
Facing targets - Ever get annoyed (especially melee classes) when an enemy goes behind you? Especially in pvp when rogues move behind you. Might be more viable if you use a mouse a lot to turn, but if you are more of a key presser, just spam you interact with target key. This also makes you run to the player if they are out of distance.
Open NPC dialog box - Ever been at a crowded NPC and not be able to click it? AH? Flight path? Handing in when 10000 people on mammoths are covering quest giver?
Can interact with NPC's through walls - You can stand by the mailbox in SW outside AH entrance and access the AH at the same time. Saves ALOT of time from running
Avoid being kited - Silly druid/hunter kiting you? Spam your interact with target binding and it will "right click him", following him and taking the shortest path. You will stick on him like flies on shit :P
Boss fights - Those that involve you moving quickly to a target. An example is to get the achievement in (first boss). Random party members get Web Wrapped and you must break them quick. Could do /tar + instant cast spell, but not all classes have this.
And many more. Will try keeping this updated. Feel free to add some more yourselves.