Just a simple tip, since the game is still pretty fresh and many of you might have not noticed yet that this is an option - Enable "Auto Face Target" under "Combat" settings, and you won't have to worry about aiming at mobs any more! (it works only in PvE).
Once enabled all you'll have to do to aim at mob is to simply click it and shoot / cast your spell (the same way as you had to do it in WoW).
Enjoy!