All those wanting to main a DH are going to be sorely disappointed when people start saying "LF DPS (no DH)" or guilds refuse to recruit DHs because they have too many already. I saw it with paladins in TBC on horde, I saw it with DKs in WotLK. Should be entertaining to see how many posts hit the forums after Legion goes live with people complaining about how they can't do anything on their DH because fights aren't melee friendly or no one wants a DH because there's too many already.
I intend to roll a DH (if I should even get the expansion) but I'll stick to my shaman. DH will just be another FotM class that will last maybe a patch before people start to gravitate back to their old mains. Only a lucky few as well as the stubborn will stick with the DH just like they did the DK.