Were there any changes to aggro mechanics in WoD

I haven't played much WoW since Cataclysm, so I'm not sure if anything has changed. I rolled a Resto Druid a couple of weeks ago, and in every dungeon I'm queued for I have found myself pulling all aggro off of the tanks. At first I thought that maybe it was just a couple of bad tanks, but it has been every single one that I have encountered.

It has gotten to the point where I nearly have to let the group wipe before I start healing in order to keep the aggro on the tank.

Is this typical? Do healing spells now generate significantly more aggro? Or has every one of the last 60 dungeons I've encountered just had a tank that wasn't as effective as I remember?

There were no threat mechanic changes in WoD that would have caused the problem you mention. In fact threat generation was significantly buffed (thanks Troyen for the link) for tanks.

You did mention in the comments that you are currently level 57 and with that in mind it is important to note that the game designers do not balance the classes and mechanics too heavily at anything but max level.

That being said I would say this is likely just caused by inexperienced tanks not being able to hold threat on multiple targets (I know some tank specs get their best aoe abilities pretty late).

I would also say that as long as your gear is up to par it shouldn't be an issue* just keep healing and stay near the tank so he can taunt anything you pull back.

*Once you get closer to max level this will no longer be the case.