I don't think it's melee characters hitting you from 15 yards away more than it's the fact that once the opponent has "locked" into it's attacking animation (ie got into the minimum attacking range to start it's attack up) it will hit you. The only way that attack will not hit is if it's blocked, dodged or if you used an evasive skill on time. Running away and creating a gap won't do you any good, the gap can be 1 yard or 100 yards.
The way I play this game now is that any time an enemy has gotten close enough to me and has started it's attack animation, I immediately dodge or SS out. I even have Evasive Fire so even if they get in close enough for melee attack I dodge away before they do anything.
But yeah you are screwed against "fast" elite mobs. They will just keep attacking you until you run out Discipline/Spirit or whatever the hell your character uses to escape and will just rush you down until you die.
Blizzard has confirmed your suspicions; If you're in range when the animation starts, it's going to hit. They've designed it this way because "dodging isn't fun" or some such BS.
I think they want you to use the evasive skills on your character to avoid an attack. To be honest, against a DH most regular enemies shouldn't be able to touch her no matter the difficulty or the enemy type. Things get complicated with Elite mobs in Hell/Inferno who can't be slowed that much, can't be stunned and can have movement speed faster than you along with teleports, jailing, walling and vortex abilities.
In that regard the whole "locked" in attacks aren't balanced at all especially when some enemies pop out of no where on the map like those ghosts in act 1 or those teleporting Nagas in act 2.
