As a warlock if you get the right spec (affliction/dm), you can kill players of the opposit faction in mid air, this is very cool as the killed player will be unable to ress so he must receive 15% debuff and ress sickness
I RECOMMEND EPIC FLYING MOUNT
It's easy to do it but you must watch out that you don't die instead <.<
If you see a player hanging somewhere in mid air, maybe he is afk maybe he is not, it would be quite difficult to kill a player unless your a pyro mage with slowfall -.-, fly below the player untill your siphon life, curse of agony and corruption can hit them, dismount and cast them (this will take approx 4 sec, so you must be atleast within 4 sec falling limit of the player you wish to kill or you will fall to low and it will fail allerting the player so he will fly away)
Ofcoure you will fall and take dmg yourself, so make sure you don't go to high or it will be SPLAT!, so don't think on doing this method with players that are so high if you go up to them you can't see the ground...
Your dots will do around 10k dmg (with sacced succubus and heroic/karazhan gear) and will kill players, most players don't understand it and fly even higher! So then they will surely have to take ressurection sickness
If you don't happen to kill the player and you really hate that sacrifice your voidwalker next time, your dots will do around 1500 less total damage but when you fall banadage yourself and fly back up, your health will be back to full and cast your dots again, even if the player will fly away you will most likely get a second chance to cast your dots again, believe me i did it lots of times
You can also do this with players flying by really low over the floor when you are playing, just cast your dots they will most likely die XD, if not mount up and re-cast even if the fly higher make sure you don't die if you fall.