It's a big issue in tanking. Tab targeting is limited to enemies in your view, which is both a blessing (it's controllable to some degree) and a curse (when a stray wanders out of view and you can't tell where).

Pointer click-targeting is pretty bad with all the epilepsy-inducing special effects doing a pretty good job of hiding your pointer (especially when you have 5 other people causing sparklegasms all over your primary tank target)

It's something you're going to be fighting quite a bit. I can only imagine how horrid it would be to be playing on a controller and having to contend with the frantic early stages of getting threat.