Normally I try not to jump to conclusions about Blizzard's decision (as they generally are more experienced and have spent more time thinking about Wow than I have). However, when it comes to the new Call to Arms bonus, I feel like Blizzard is missing an obvious problem with the new call to arms math...
Tanks already have a huge advantage when it comes to queuing for heroics. Instant queues (instead of the 45 mins it was for dps today) plus it is easy as a tank to find a dps who will pay you 50-100g for your instant queue. I can only conclude, therefore, that the bonus of flasks or gold will not provide any additional motivation for tanks to queue. That leaves pets and mounts as the primary motivation.
From everything I've seen when it comes to mounts versus pets everyone is going to care about the mounts. After all most of the pets can be bought at the AH for relatively cheap if you really cared. So the real draw will be the chance to get one of three epic mounts. But here is where the math doesn't work.
Let's say the drop rate of a given mount from Call of Arms is the same as the drop rate from running an old instance for it. This would be a 1% drop chance. Sounds pretty low until you realize many players run over 100 heroics per expansion. With a 1% drop chance this means you would average over 1 mount per expansion, meaning essentially every tank would have at least 1 of the three epic mounts. Well if every tank has it then suddenly its value (which is mostly how rare it is) is gone.
So lets say blizzard realizes this and drops the drop rate so that mounts are still rare. Suddenly we have a drop rate more like .01% (so that more like 1-5% of people will have the mount). Well let's image that you are a dps right now and you are considering tanking. You haven't done it thus far because tanking is more of hassle, you don't have the gear, etc. You haven't done it despite the fact it would make you gold and save you tons of time. Are you really going to change now if there is only a .01% chance you will get anything out of it?