Mind control no longer breaks on ticks. If the initial cast is successful, it will last until the spell expires or the channel is disrupted. It doesn’t break with distance, either--you can walk somebody far out of cast range, around corners and off cliffs. Be aware you can end up with an evading mob this way. I once MC’d a mage with slowfall and rocket boots off the AB cliff, and the channel lasted until he splashed down in the lake south of the stables. After that I hotkeyed a cast cancel so I can break the spell without clicking around.
A priest does need hit to make sure that the initial cast lands. This is not a problem if you have the talent Twisted Faith. In my Holy build I put six points in Shadow, giving me 3% extra haste, a shorter cooldown on shadowfiend, and one point in Twisted Faith, which means half my spirit counts as hit. That’s more than enough to hit heroic trash, and it also helps my shadowfiend hit bosses. It’s also useful on the handful of fights where you can shackle undead. I took it so my Chastise and Psychic Scream would always hit in PVP, but I’m pleased with its utility in PVE. On the other hand, I’m missing out on 6% mana reduction on instant casts.
When mind control breaks, the mob will head straight for you. You can Fade so the tank can pick it up easily if it’s that mob’s turn to die, or if you plan to renew the mind control you can use Chastise (as Holy) or Psychic Horror (as Shadow) to stop it long enough to cast the spell again.
As long as we’re talking about PVE crowd control for priests, I’ve noticed that in most heroics, trash packs are spread out enough that even unglyphed, Psychic Scream can be used safely. It comes in really handy in heroic Halls of Origination, especially on the Setesh fight--you can scatter the Void Sentinel adds chasing your tank, greatly reducing incoming damage.