If you've ever selected your own character or another player, you'll see there is a blue circle around their feet. There is also a 90 degree area where the circle doesn't appear, this is known as being hit in the bad place getting hit there increases damage taken and chances of critical hits from what I've seen, effectively the same as any mob you attack having that same circle with the space. Proper placement and direction of mobs is good practice, proper placement of yourself as a tank should be essential.