After reading comments and assorted blog posts about Call to Arms, here are some more thoughts.
Tanking isn't Quantum Mechanics
Gevlon thinks that tanking is too difficult. Difficult enough that offering an incentive won't improve things.
On the other hand, he's offering 1000 gold to everyone present at a guild-first kill of Nefarian, which seems to imply that you can motivate people to complete difficult tasks with rewards. On the scale of things, killing Nef is much more difficult than tanking.
First off, tanking isn't that hard. It looks hard at first, but practice improves everything. Personally, I think that if more people step up and try to tank, more people will find that it really isn't as bad as they thought it would be.
(Except for Ozruk. You're on your own there.)
Second, the reward is also aimed at experienced tanks who have finished gearing up in heroics. If the reward pushes those tanks to do just two daily heroics instead of one, it's a major win.
Tanking Shortage Is Not Caused By Raid Slots
I used to think that part of the reason tanks are rare is because 25-man raids require fewer than 20% tanks.
But the truth is that 25-man raids are dying (a topic for another time), and the vast majority of raiders are in 10-mans. And 10-mans have the same 20% tank ratio as a 5-man group.
So I'm not really sure this reason is valid anymore, if it ever was.
The thing about PvE in WoW is that it tends to be very egalitarian once you're in a group. There's a sense that everyone is part of the group, and everyone is contributing more or less equally, and thus everyone has more or less the same shot at the rewards. This may not be strictly true, but that's how the default loot systems work. Even Need before Greed is strictly based on armor-type, rather than any performance-based quality.
This Call To Arms explicitly breaks that illusion. Now, though the group completes the task together, one member of the group gets singled out for an extra reward. And going against that egalitarian grain often rubs people wrong.
For example, a lot of the high-end guilds give loot priority to their tanks and healers. This is deliberately unfair to the DPS, but the DPS accept it because it helps the group progress faster. The DPS sacrifice for the group, and that can restore the cohesion.
But in Call to Arms, there is no external group for the DPS to invest in. I think this will be the biggest challenge for Call to Arms. Tanks often feel "abused" by the dps and healers. Will it get worse if the game deliberately sets them apart and gives them extra rewards?
Envy is a deadly sin, but it still causes damage.
The Value of Experimentation
I may have mentioned this before, but I think our society is becoming too risk-averse in many ways. What's wrong with running experiments, with trying something new?
Honestly, if Blizzard had floated the idea of the Dungeon Finder before it was available, I think everyone would have enumerated all the things that could possibly go wrong and insisted that the Dungeon Finder would be a failure.
And a lot of those negative effects did happen. But they were greatly outweighed by the benefits, and the most negative behaviors corrected where they could be.
Doing something, seeing what happens, and fixing the problems as they emerge is far more likely to produce advances than trying to theoretically construct the "perfect" system beforehand.