I’m pretty sure a lot of people already know about this – or at least, Alliance-side AH players would, from the AH-from-near-the-mailbox trick that was then obsoleted with the Stormwind revamp in the Shattering. Still, it’s quite a useful trick and I keep running into people who don’t know about it (to the point that I got accused of exploiting while I was running around Orgrimmar doing that one cooking daily, because I was picking up the crawfish without stopping).
In the keybinds, under “Targeting Functions”, you will find an option called “Interact with target”. By default, it’s unbound. Bind it to something convenient and easy to remember (I use the F11 key). It works exactly the same as a right-click on anything you can actually target.
So, for example, you can pick up the crawfish for the Orgrimmar cooking daily (or the Alliance equivalent, of course – or the irritating Chicken Escapees, or the Snowfall Glade Pups) simply by targeting them (nameplates make this really easy), running or swimming up to them and mashing whatever key you have bound to “Interact with target” – you’ll loot without stopping.
Trying to use a vendor who just won’t stop moving? Mash the key.
Trying to loot that one boss you just killed in an instance you outlevel amidst a sea of sparkling corpses? /target to select and mash the key. Same thing if you’re dealing with players standing on a NPC so it’s impossible to click (seasonal vendors come to mind), or NPCs aren’t loaded yet and you’re in a hurry (assuming you remember the name of the NPC).
It’ll even start your autoattack when you’re targeting a hostile mob – not nearly as useful as the other cases, since you can just mash any of your offensive abilities to do that, but still.