How It's Gonna Happen
We're probably going to see these changes all implemented in patch 4.0, which will come out a bit before Cataclysm is released. This is how it worked for the last two expansions. The stats on your current gear will change -- you will not be stuck with useless Int or ArP. Blizzard has actually done a big gear change like this before (and more on that in a bit).
I know that in general we are built to fear change, and a lot of hunters hate the thought of the change from mana to some kind of focus. We don't know yet exactly how this is going to look or work, so let's withhold the judgment and QQ on that until we get to the open beta and actually see how it will happen. It may be no more of a re-learning of our class than we had to do in the transition from vanilla to BC.
Agility = 2 Attack Power
For those grognards out there like me, you'll remember that back in the early days 1 agility was worth 2 attack power. And that was fantastic! Grandpappy Frostheim can tell you how much simpler gear choices were back then. Then they made the change to agility only giving us 1 AP, and reworked all of our gear to have AP on it. So this kind of wholesale changing of every piece of hunter gear in the game has happened before, and the end result worked out quite well for us. We were not screwed by the process.
We're still going to have to pay attention to what our AP actually is, and our shot formulas are still going to be calculated off of our AP, but this is likely to make agility an even stronger stat than it is now. However, they are reducing the crit component of agility to compensate. Clearly Blizzard doesn't want the vanilla situation where agility was so good it was basically the only stat we cared about.
Also, I wouldn't be surprised to see that we still have AP enchants for certain slots.
Haste is the New Armor Penetration
I'm predicting that haste is going to be our new super complicated stat. If it does indeed increase our rate of focus generation, we're likely to see certain haste plateus where haste becomes much more useful. So for example if you have 0 haste your rotation is A - B - C - C - C, and getting another 50 haste doesn't change anything. But suddenly you get to 51 haste and your DPS can jump up by changing your rotation to A - B - C - C.
The interesting thing about having haste increase our focus regen is it really moves it into a more viable role of letting us shoot more, and we likely won't have a soft cap on our haste any longer. However if the haste plateus that I fear happen, that'll be a lot more complicated than a soft cap.
It's also worth noting that you will never see a crit chance as high as what you will have in Wrath, ever again. Most hunters have a base crit change over 50% at the moment, with some even going over 60% -- and that's before raid buffs and shot-specific talents.
The point when a crit is more common than a hit is when things are getting kinda weird, and I'm not surprised that Blizzard wants to rein this in. We don't know what they intend; however, I'd predict that they want a well-geared hunter to have something like a 35% crit chance, perhaps as high as 40%, based on what we saw in past expansions.
Extreme Soloing and Pet Tanking
A lot of these changes imply intriguing things for extreme soloing and pet tanking. We're going to have stamina levels very close to those of plate guys. Our armor is going to be closer to that of plate. Even if nothing else changes, that alone will be a huge boost to our tank pets. But we can also add on to that the previous promise to have every stat transfer to our pets in some percentage, and we can imagine stacking dodge and parry just to benefit our pets and filling in as off tanks in our raids.
However, we're also seeing the crit reduction from resilience being removed, which is currently necessary to make our pets crit immune. Furthermore, I'm pretty certain that Blizzard never wants hunter pets to be truly viable raid tanks. We aren't a tanking class, and the last thing they want is for our pets to take a tank slot from an actual tank class. After all, how would you feel if a warrior or DK could take a DPS slot in your raid?
It's All Subject to Change
We always need to keep in mind that this stuff is all subject to change. The only thing that's really certain is that some of it will change, in some way. Furthermore, we still have stats like Mastery that are a complete mystery, even more so than how focus will work for us.
There are a lot of variables in the Cataclysm hunter equation that are still completely unknown to us, and until we have all that information, we can't make a call on the big picture of what our class will look like.
I can tell you that I'm happy and excited about many of these changes. I'm hesitant about haste for fear of the complexity that it could bring -- Blizzard wants simple to understand but with a great depth, and I think this is one stat that may not be intuitive. And finally, I'm completely withholding judgment on the mana to focus change until I can see what it's actually like.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.