I reproduced the explanation from the Dev Watercooler because I wanted to highlight how this works, and more important, how it is still a stat which can be tuned. On the face of it, 1% Versatility is incredibly simple - just as 1% Critical Strike Chance increases your chance to critically hit by 1%, 1% Versatility increases your damage, healing and damage absorption by 1%, which also reducing your damage taken by .5%. How does this differ from the proposed effect of Amplify? Why, in other words, is Versatility a worthwhile trade for the previously proposed stat?
Versatility is pretty simple: 1% Versatility grants a 1% increase to your damage, healing, and absorbs, and reduces the damage you take by 0.5%. It's a straightforward, obvious upgrade to your primary role's performance, but also gives significant boosts to secondary role performance and survivability. The healing increase it provides does work on self-heals, such as Recuperate, for example. We won't be tuning it to be anyone's highest throughput secondary stat, but it'll be close, and it'll give you a nice boost to how versatile your character is in the process. It'll be especially attractive to hybrids who want to feel more "hybridy."
First, let's look at how Amplify worked.
X% Amplify means: +X% Crit Damage Multiplier and Multistrike Damage Multiplier, and +X% more Haste/Mastery/Readiness/Spirit/Armor from gear.
So we can immediately see that Amplify would have increased our damage, but it would have done so by increasing pretty much every secondary stat we had - in essence, Amplify becomes the stat that is all stats. It would have to be tuned with exquisite precision. Too good, and it replaces practially every stat in the game. If you want more Haste, stack Amplify. If you want more Mastery, stack Amplify. If you want more Spirit or bonus Armor, stack Amplify. But if it was tuned poorly, then it becomes useless - if it doesn't offer a compelling boost, you might as well ignore it and stack X instead. And by tying it to critical strike damage it inflates the value of critical strike as a statistic - you want Amplify to boost those crits and give you more haste and mastery, and then you want crit. It's a potential land mine of unintended consequences. It's fine when you have it on one trinket - it can't unbalance anything if you can't stack it. But when you can, the cracks start to show conceptually, and even in the original post I did about it I mentioned that Amplify was a stat that magnified other stats, a kind of 'meta-statistic' that sounds kind of awesome, but also kind of complex.
We're trying to simplify gear. Making a stat that plays threshold games with your other stats does not simplify anything. Readiness, for its part, was even worse - in order to make it a compelling DPS/Healing/Tanking stat you had to try and balance every single class cooldown in the game against each other. This is a really, really hard thing to do, and it takes a giant steamroller to the idea of class difference. Having one class have a cooldown that increases a specific kind of damage while another just increases crit chance works, when you can balance them out in terms of how long their cooldowns are. But when Readiness starts shrinking all those cooldowns? You have a balancing nightmare on your hands. Add in healing and tanking cooldowns and it gets even worse.
Now, is Versatility complex? Absolutely not. You have 1% Versatility, you do 1% more damage and healing and absorbs and you take .5% less damage. It's a solid stat for anyone - DPS, healer or tank (especially tanks with self-heals or absorbs, which is pretty much all of them). It's easy to grasp, easy to tune (just tinker with how much rating it takes to get 1% and you're done) and easy to balance. It's not as viscerally compelling as Amplify is, nor as exciting sounding as Readiness with its cooldown reduction, but that just means you don't have to sit around and try and figure out if it's better than another stat or not. You get a piece of gear, it has Versatility on it, and you know what it does and how much it does it.
Finally, as has been pointed out, Versatility is a dream stat for hybrids who might want to change roles and play up their hybrid strength. Since all healers are hybrids, it means that a healer who needs or wants to off-tank or throw out some damage has a bit more oomph in those departments without becoming unbalanced and overpowered. It still provides benefits even for pure DPS classes (rogues will like the damage boost, healing boost for their self-heals, and damage reduction) but it definitely offers some utility to classes like a warrior who is switching from DPS to offtank for a specific boss, or a paladin tank going healer for a fight.
It's not a complicated change, but it should work out better in the long run. I'll miss Amp as a stat, and I hope we see it again on items. But I'm interested in how Versatility will work for people who play different roles, and whether or not it will work in PvP.