Here's the link.
http://www.sageadvice.eu/tag/reaction/
Crawford says no, Mearls says yes (a short way down the page, October 22nd).
Which is it according to the PHB? I am waiting for mine in the mail but I have a sneaky suspicion this exact scenario will come up during tonight's session because our wizard just got Misty Step, already has Shield, and the group is facing Magic Missile wielding maniacs tonight.
The thing to keep in mind is that "round" and "turn" are two different things. A round is an entire initiative cycle. A turn is an individual combatant's segment of that cycle. So if you have a fighter, a wizard, a rogue, and a giant, each round would consist of four turns--one turn for each combatant.
The rules on spellcasting say that a bonus action spell precludes casting any other spells on that
turn. What this means is that you cannot cast
misty step and
shield on the same turn. But you could cast
shield later in the round.
As an example: Let's say you're playing an Eldritch Knight. You win initiative and use
misty step to get next to an ogre. You then whale on the ogre with your sword, and move away using your regular movement. Moving away provokes an opportunity attack from the ogre. Because it's still your turn, and you've cast
misty step already, you cannot cast
shield in response to the OA.
You take the hit and end your turn. Now the ogre moves up to attack you. It's still the same
round, but it's no longer the same
turn, so you can cast
shield against the ogre's regular attack.
As I read it, this is the disagreement between Crawford and Mearls: Crawford was applying the rules as described above, whereas Mearls was saying he'd allow you to use
shield against the OA. Crawford is correct by RAW, but I think Mearls's ruling makes things simpler and more logical, and I'd probably go with it unless someone could point me to a way it could be abused.